What are the differences between P21 and SFC in civil engineering CAD? Four criteria for when to use each

In civil engineering CAD practice, how you hand over drawings, how you check them, and how you link them to the next process is more important than the act of creating the drawings themselves. Especially before delivery or when sharing with subcontractors, the amount of rework can vary greatly depending on which format you use even if the drawing content is the same. A common example often compared for this purpose is P21 versus SFC.

Many practitioners searching for P21 in civil engineering CAD are not merely curious about the difference in file extensions; they want criteria for judgment such as which to choose in which situation to be safe, which is better for editing, how much attribute information can be retained, and which will avoid problems during inspection and delivery. What is really needed on site is not just the definition of formats, but the axes for choosing between them.

To conclude up front, while P21 and SFC may appear to serve similar purposes, the points emphasized in practice differ. P21 tends to be better suited for exchanges that consciously include attribute information and inspection support, preserving more information in the exchange. SFC, on the other hand, is easier to handle when you want to exchange drawings reliably in terms of geometry; its strengths tend to be ease of viewing and sharing. However, this is not to say one is always superior. When you think in terms of the four axes of delivery, intermediate sharing, reuse, and verification tasks, the choice becomes much clearer.

This article organizes the differences between P21 and SFC from a practical perspective for civil engineering CAD practitioners. It explains in an easy-to-understand way which to choose to avoid stopping work, including attribute information, compatibility, ease of editing, inspection handling, and differences in handover.

Table of contents

• Understand the differences between P21 and SFC first

• How to think about the information that can be handled by P21 and SFC

• Criteria for choosing between them for delivery

• Criteria for choosing between them for intermediate sharing

• Criteria for choosing between them for reuse

• Criteria for choosing between them for verification tasks

• How to view differences in attribute information and compatibility in practice

• Differences in ease of editing and ease of handover

• A way of thinking to avoid confusion in inspection handling

• How to proceed so you don't make wrong judgments in civil engineering CAD practice

• Operational thinking that looks ahead to on-site data utilization

Understand the differences between P21 and SFC first

P21 and SFC are both known as formats that appear when exchanging drawing data in the civil engineering field, but their practical character is not the same. Broadly speaking, P21 is more suitable for exchanges that consider not only the appearance of drawings but also the information attached to shapes, while SFC tends to be easier to use when you want to represent drawing geometry stably and make handover and viewing easier.

What is important here is not that practitioners memorize the format names themselves. It is to understand in which situations choosing which format will make downstream processes easier. For example, for data submitted to clients or inspectors, it is important that information can be checked later. Meanwhile, for internal or subcontractor intermediate sharing, it may be more important that the drawing does not collapse visually and that the recipient can open it easily.

In other words, the difference between P21 and SFC is not simply a file format difference; it is a difference in what you preserve and what you prioritize when handing off data. Whether you prioritize fidelity of appearance, inspectionability including attribute information, or ease of editing will determine which format to choose.

In civil engineering CAD practice, drawings are not finished once created. As design, construction, final shape confirmation, coordination, and delivery progress, the required properties of drawings change. Correctly differentiating when to use P21 and SFC is less about understanding formats and more about making practical judgments that keep drawing operations from stalling.

How to think about the information that can be handled by P21 and SFC

To understand the differences between P21 and SFC, the first thing to grasp is what information you want to include when handing data over. Civil engineering CAD drawings may include not only visual information such as lines, text, and symbols, but also invisible information such as the meaning of line types, the roles of layers, attributes of shapes, and ancillary information about components or elements.

P21 is often conceived with the premise of handling such non-visual information as well, making it well suited to situations where you want to pass drawings while preserving some of the meaning of the drawing structure rather than merely handing over drafting data. For example, when you want to check the contents of elements later during inspection or verify consistency while referencing attributes, the P21 approach becomes useful.

On the other hand, SFC is often handled with an emphasis on display stability and ease of exchange as a drawing. Its strength lies in ensuring the drawing does not collapse visually and that the recipient can receive the same geometry. In other words, SFC is often more compatible with workflows that focus on exchanging the drawing itself rather than leveraging attributes heavily.

Misunderstanding this difference can cause two kinds of failures on site. The first is operating mainly with SFC when you actually want to retain attributes, resulting in insufficient information for later reuse or inspection. The second is using only P21 for intermediate sharing, making editing or checking difficult for the other side and increasing the work of converting formats later.

Therefore, P21 and SFC should be considered not in terms of superiority but as differences in how much information you preserve when handing data over. Do you want to convey the meaning behind shapes, or do you primarily want to ensure the drawing is reliably shared? Clarifying this difference first will reduce indecision later.

Criteria for choosing between them for delivery

Of the four axes, delivery is the one that should be considered most carefully. For delivery, priority is given to the recipient’s ability to verify, inspect, and reproduce data rather than ease of work. For that reason, you need to choose P21 or SFC based on how much information the recipient needs.

P21 tends to be more suitable for delivery when you want to submit drawings while preserving attributes and structure. If it is important that the meaning of elements can be traced after submission, that the data can be easily checked during inspection, and that the drawing’s consistency as data is verified rather than just matching visually, P21 has high value. Particularly when you want to preserve ancillary information within the format, P21 is easier to justify for delivery.

Conversely, if the recipient mainly values drawing display and prioritizes viewing ease and stable display, SFC can be easier to handle. If the recipient mainly opens the drawing to review content and does not require detailed attribute utilization, receiving SFC may impose less burden on them.

However, it is important not to adopt a rule such as “delivery always means P21” or “delivery always means SFC.” The answer depends on whether the delivery requirements and operational rules demand a visual check or an inspection that includes attributes. In practice, choose the format according to the recipient’s verification perspective.

Also, for delivery, do not judge solely by how the converted file looks. Even if attributes remain in P21, you should check for any visual inconsistencies after conversion; and for SFC, even if the appearance matches, you must confirm in advance that necessary information is not lost. In other words, the verification criteria before delivery differ by format.

A common delivery mistake is submitting in the same format used during work simply because it was easy to handle during the process. What is easy internally may not be suitable for the recipient’s inspection. The delivery standard should be determined by the recipient’s ease of verification, not by the creator’s convenience.

Criteria for choosing between them for intermediate sharing

For intermediate sharing, priorities differ from delivery. For design reviews in progress, internal reviews, exchanges with subcontractors, and reflecting correction instructions, it is important that the recipient can open the file reliably, that the drawing does not collapse visually, and that exchanges are fast. For that reason, the ease of use of SFC often comes to the forefront for intermediate sharing.

SFC is well suited to intermediate-stage handovers in the sense that it makes stable sharing of drawings easier. If the recipient is not aiming to edit detailed attributes but mainly to check display content and identify correction points, SFC is often sufficient. In particular, when you want to hold a meeting to look at shape and annotations together, being able to open and view the file immediately is important.

On the other hand, even for intermediate sharing, if you plan to re-edit the file as-is later, hand it off to another person while preserving attributes, or manage correction ranges using element information, P21 may be more appropriate. Intermediate sharing does not always mean a simple format is acceptable. You must consider what will happen after sharing.

A common practical failure in intermediate sharing is that an SFC intended for viewing is used as if it were the source for corrections. In that case, when re-editing occurs the attributes may be insufficient, or the meaning of shapes becomes diluted and the original intent is hard to convey. Conversely, if the data is intended for editing but P21 handling is burdensome for the recipient, confirmation may be stalled.

Thus, for intermediate sharing it is important to decide in advance what the recipient will do: only view, mark up, edit, or pass it to the next process. The choice of format changes depending on whether the sharing is for showing or for using. Not blurring this distinction is the key to preventing confusion in intermediate sharing.

Criteria for choosing between them for reuse

The difference between P21 and SFC is most strongly expressed in reuse. Drawings are not necessarily finished once delivered. There are many situations where they will be used again later: future renovations, maintenance management, quantity checks, revising construction plans, or applying to similar projects. At that time, how much information remains preserved makes a big difference.

If you plan for reuse, P21’s value increases. When attributes and structural information linked to elements are useful, a format like P21 that can handle richer information is advantageous. For reuse, it is more important that the meaning can be read later than that the appearance is merely the same.

For example, if it is unclear whether a certain shape is just a line or an element for a specific purpose, it becomes difficult to decide how to reuse it. Even if text and symbols are visible, if the underlying meaning is lost, reuse later requires someone to interpret each item manually. This increases work time and the risk of misunderstanding.

SFC is not unusable for reuse. However, depending on the depth of reuse, its limitations become apparent. If you reuse the drawing as a reference document, check only the overall geometry, or archive it as a past drawing, SFC can be sufficient. But if you want to leverage attributes for the next processes, lack of information often becomes an issue.

A common mistake from the reuse perspective is assuming that because delivery or sharing went smoothly, the format is also suitable for future use. Short-term handover and long-term reuse require different quality of information. A drawing that is fine to view now may show significant differences depending on whether attribute information is present when used six months or several years later.

Therefore, if you plan for reuse, it is important to manage data so that information is preserved, separately from the submitted format. Rather than standardizing on either P21 or SFC, it is realistic to separate original data management for reuse from derived data management for handover. Practitioners should understand the importance of this two-tier management.

Criteria for choosing between them for verification tasks

For verification tasks, whether P21 or SFC is appropriate depends on what you want to verify. If you want to check the drawing’s appearance, positional relationships, presence of annotations, or print image, a format like SFC that stabilizes display is easy to use. If you want to check attributes, element meanings, data structure, or the consistency of drawing information, P21 can be easier for verification.

Verification tasks in civil engineering CAD are not limited to visual checks. There are many targets for verification: the organization of layers, classification of elements, information affecting quantity calculations, and attributes you want to cross-check during inspection. For that reason, treating drawing verification and data verification as the same task often leads to incorrect format selection.

For example, if you are doing a screen check before a meeting, priority is that the drawing opens and the parts you want to see are visible. In that case, SFC is convenient. However, for pre-submission consistency checks or pre-inspection self-checks, visual agreement alone is not enough. You must confirm that attributes have not been lost, that element information has not collapsed, and that the data can be passed on in the expected state. In this stage, P21’s verification value increases.

A common failure in verification tasks is equating a viewing check with being ready for inspection. If the drawing opens, text is visible, and lines are displayed, one may mistakenly judge that everything is fine and overlook inconsistencies in attributes or structure. Conversely, overemphasizing information checks can complicate everyday sharing to stakeholders and stall the verification process.

Therefore, a practical rule of thumb is: use SFC for appearance checks and P21 for information checks. There are exceptions depending on the project, but deciding what to verify in advance reduces confusion in format selection.

How to view differences in attribute information and compatibility in practice

When comparing P21 and SFC, attribute information and compatibility are often discussed. However, these two should be understood as separate issues. Attribute information concerns how richly meanings are preserved, while compatibility concerns how stably the recipient’s environment can handle the data. In practice these are often conflated, but their priorities differ.

P21 often has an advantage in handling attribute information. It is suitable when you want to handle what elements in the drawing mean and what ancillary information they have beyond appearance. Therefore, in operations that later reference attributes or perform information cross-checking, P21 tends to offer more benefits.

In terms of compatibility, however, the recipient’s environment and operational rules play a part. Even if a format can theoretically hold more information, if the recipient cannot handle it properly the practical value is lost. Conversely, SFC may provide more stable viewing and exchange in practice, leading to trouble-free transfers.

It is important not to judge compatibility merely by whether a file can be opened. Being able to open a file and being able to use it as expected are different things. A file may be viewable but not editable, visible but with attributes inaccessible, or printable but hard to reuse. Practitioners need to consider compatibility through to task completion.

As a basic guideline, emphasize P21 if you prioritize attribute information, and emphasize SFC if you prioritize stability of handover. However, rather than covering all processes with one format, using formats according to purpose reduces compatibility troubles. Choosing a format should be considered as assigning roles by process, not as a performance comparison.

Differences in ease of editing and ease of handover

One dilemma practitioners face when choosing P21 or SFC is whether to prioritize ease of editing or ease of handover. These two may seem similar but can actually conflict. A format suited for editing may not be ideal for handover, and a stable handover format is not necessarily suited for re-editing.

P21 is favorable for re-editing and information utilization because it preserves information, but it requires caution depending on the recipient’s environment. Because it assumes editing, recipients who only want to confirm display may find it somewhat heavy. In short, data optimized for use is not always optimal for showing.

SFC is easier to integrate into a workflow of handing over drawings for confirmation and is often easier to handle for routine sharing. On the other hand, it can feel lacking when deeper editing is required, depending on the type of information it preserves. If you want to leverage the meaning of shapes, display ease alone is not enough.

Therefore, in practice it is effective to clearly separate the editing original and the distribution data. Manage the source data used for editing in a format that preserves information such as P21, and use SFC for review and distribution. This way you can leverage the advantages of both. Relying on only one format will cause problems somewhere.

Also, editability is not only about the feel of the software; it also concerns whether you can read the meaning later. Being able to move a shape does not necessarily mean it is easy to edit. It should be easy to decide what to fix and to narrow down the correction range by checking attributes. From that perspective, P21’s value goes beyond a mere storage format and affects the efficiency of later editing tasks.

A way of thinking to avoid confusion in inspection handling

The area where confusion in using P21 and SFC most often leads to trouble is inspection handling. In inspections, what matters is not visual impression but whether the data has been submitted in the requested state. Therefore, deciding the submission format solely because it worked for everyday sharing or viewing is risky.

When considering inspection handling, first clarify what will be checked. If the central concern is that the drawing displays correctly, that points one way; if elements’ attributes and structural information will be verified, that points another. In the latter case, the importance of formats that preserve information such as P21 increases.

If inspectors emphasize readability and stable display, SFC’s advantages may be decisive. In short, inspection handling is not about always choosing P21 but about choosing the format that matches the inspection perspective. Getting this wrong can lead to re-conversion or replacement after submission and halt work.

For inspection handling, it is important to separate post-conversion checks by format. For P21, verify how attributes and information remain; for SFC, focus on display status and whether anything is broken. Using the same checklist for every format will not capture format-specific risks.

Also, before inspection, organize not only the submission data but also the source data and work history. This makes it possible to trace which state was used for re-output if corrections are requested. Whether you choose P21 or SFC, don’t rely on a single final submission; organizing check copies, retainers, and re-output copies is what true inspection readiness looks like for practitioners.

How to proceed so you don't make wrong judgments in civil engineering CAD practice

As shown so far, the difference between P21 and SFC is not a simple format comparison but a difference in suitability to process stages. To avoid confusion in practice, first decide which process the data will be used in, and then organize the required amount of information and how the recipient will use it.

As a way of proceeding, it is important not to restrict the data’s role to a single purpose from the start. Civil engineering CAD drawings serve multiple roles: creation, sharing, verification, delivery, and reuse. Because the appropriate format differs by role, avoid trying to complete everything with a single file.

Next, separate originals and distributions. Manage originals so they lose as little information as possible, and choose formats tailored to recipients for distribution and verification. This operational approach allows you to use both P21 and SFC without strain, balancing ease of sharing and ease of reuse.

Moreover, standardizing the verification points before and after conversions is effective. If you separate the points to check before delivery, before intermediate sharing, before reuse, and before inspection, you will reduce indecision in format selection. For example, prioritize checking for display collapse before intermediate sharing and checking for attribute preservation before reuse.

What matters for practitioners is not that they know P21 or SFC individually, but that they can decide when to use which to minimize rework according to the project flow. With this perspective, format differences become a practical tool for operational improvement rather than difficult technical knowledge.

Operational thinking that looks ahead to on-site data utilization

When considering how to use P21 and SFC in civil engineering CAD, it is important not to think of drawing data in isolation. Recent practice increasingly involves operating drawings together with on-site positional data, as-built information, photos, and surveying results. In this flow, the choice of drawing format cannot be separated from downstream data utilization.

For example, if you plan to select P21 for inspection-aware delivery and use SFC for intermediate sharing, the role of drawings becomes clear. If the site then establishes a system to reliably perform stakeout and verification by other means, it becomes easier to link drawings and field information. Not making the drawing format bear all responsibilities and instead clarifying role separation is increasingly important in practice.

In particular, if you want to streamline on-site coordinate checks and position guidance, it is worth reviewing not only drawing handover but also surveying operations. When drawing checks and field verification proceed separately, information mismatches are likely; but with a surveying environment that is easy to use on site, it becomes easier to connect drawing results to the field.

In that sense, those responsible for improving civil engineering CAD operations should not stop at selecting drawing formats; they should also establish systems that include on-site utilization. For example, using an iPhone-mounted GNSS high-precision positioning device such as LRTK can facilitate on-site position checks and surveying tasks, making it easier to rapidly confirm drawing data results in the field. By organizing drawing operation through the P21 and SFC divide and further improving on-site verification accuracy, overall productivity in civil engineering CAD practice can be significantly increased.