ISO 10303-21-2016 PDF

Full title and description

ISO 10303-21:2016 — "Industrial automation systems and integration — Product data representation and exchange (STEP) — Part 21: Implementation methods: Clear text encoding of the exchange structure (the STEP-file format)". This part of ISO 10303 defines the plain-text file syntax and exchange structure commonly used to store and transfer EXPRESS-based product data (commonly seen as.step or.stp files) between CAD/PLM and other engineering systems.

Abstract

Part 21 of ISO 10303 specifies a human- and machine-readable clear-text encoding for representing STEP exchange structures. It prescribes the overall file layout (header and data sections), required and optional header fields, the syntax for entity instance declarations, character set and encoding rules, and markers that delimit sections and terminate the file. The standard provides a stable, widely adopted interchange format used across CAD/CAM/PLM applications to exchange product model data defined by EXPRESS schemas (other ISO 10303 parts).

General information

• Status: Published international standard (ISO)
• Publication date: 2016 (ISO 10303-21:2016)
• Publisher: International Organization for Standardization (ISO)
• ICS / categories: Product data representation / industrial automation and integration (ICS categories for product data and CAD interoperability)
• Edition / version: 2016 edition (Part 21)
• Number of pages: Typical published print edition — around the mid‑20s to 40 pages (varies by publisher formatting)

Scope

ISO 10303-21:2016 defines the clear-text encoding and structural conventions for STEP exchange files used to transport EXPRESS-based product data between systems. It covers the file-level organization (HEADER and DATA sections), mandatory and optional header entries (for example FILE_DESCRIPTION, FILE_NAME, FILE_SCHEMA), the representation of entity instances and identification tokens, delimiters and section terminators, character set and comment conventions, and rules for representing binary or external data within the clear-text framework. The scope is limited to the representation/serialization form — it does not define EXPRESS schemas themselves (those are in other ISO 10303 parts).

Key topics and requirements

• File structure: mandatory HEADER; and DATA; sections with ENDSEC; and end-of-file marker.
• Required header entries such as FILE_DESCRIPTION, FILE_NAME and FILE_SCHEMA to identify the product data and the EXPRESS application schemas used.
• Syntax for entity instance declarations, using numeric instance identifiers and explicit attribute value encodings consistent with EXPRESS data types.
• Character set, line termination, and encoding rules to ensure portability and predictable parsing across systems.
• Conventions for representing special values, null/omitted attributes, and aggregate collections.
• Markers and keywords that delimit sections and terminate the exchange file (clear textual tokens for parser reliability).
• Guidance for embedding or referencing external/binary data and for preserving metadata needed by receiving systems.

Typical use and users

ISO 10303-21 is used by software vendors, integrators, and companies exchanging 3D CAD, product structure, and associated engineering data between heterogeneous systems. Typical users include CAD/CAM/CAE/PLM application developers, systems integrators, digital manufacturing teams, quality and data-management engineers, and organizations performing long-term archiving of product model data. It is the de facto interchange file format for cross‑application transfer of STEP-based data.

Related standards

ISO 10303-21 is one part of the ISO 10303 (STEP) family. Closely related parts and standards include the EXPRESS language definition (ISO 10303-11), various application protocols (APs) such as AP203, AP214, AP242 that define domain-specific schemas, and other implementation-method parts that define XML or other encodings of EXPRESS data. Implementers usually consult the relevant AP(s) together with Part 21 to perform complete data exchange.

Keywords

STEP, ISO 10303, Part 21, STEP-file,.step,.stp, clear text encoding, FILE_SCHEMA, HEADER, DATA, EXPRESS, product data exchange, CAD interoperability, PLM.

FAQ

Q: What is this standard?

A: ISO 10303-21:2016 specifies the clear-text file encoding and structural rules for STEP exchange files — the plain-text format commonly used to transfer product model data (files often named with.step or.stp extensions).

Q: What does it cover?

A: It covers file-level layout (HEADER and DATA sections), required header fields, the syntax for entity instance representations derived from EXPRESS schemas, character and encoding rules, delimiters and section terminators, and conventions for representing special values and aggregates. It does not define domain schemas themselves (those are in ISO 10303 application protocols).

Q: Who typically uses it?

A: CAD, CAM, CAE and PLM software vendors and their customers use Part 21 files to exchange 3D geometry, topology, product structure and related metadata between systems. It is also used by integrators, data curators, and organizations performing long-term archiving of product data.

Q: Is it current or superseded?

A: The document referenced is the 2016 edition of Part 21. Users should confirm with ISO or their national standards body for any later amendments or revisions that may have been published after 2016 before relying on it for new development or compliance programs.

Q: Is it part of a series?

A: Yes — ISO 10303-21 is one part of the ISO 10303 family (STEP). Other parts include the EXPRESS language (Part 11) and numerous application protocol parts (APs) that define the actual product data schemas to be serialized using the Part 21 clear-text encoding.

Q: What are the key keywords?

A: STEP, PART 21, clear text, STEP-file,.step,.stp, EXPRESS, FILE_SCHEMA, HEADER, DATA, product data exchange, CAD interoperability.