STEP File Basics

STEP files (*.stp, *.step) are plain text files specified by ISO 10303-21.

1. File Structure (Anatomy)

A STEP file is a collection of "Instances" that reference each other. Visually, you can think of it as a book where each page (Instance) can point to other pages.

ISO-10303-21;
HEADER;
  /* Metadata: Filename, author, AP definition, etc. */
  FILE_NAME(...);
  FILE_SCHEMA(('AP242_MANAGED_MODEL_BASED_3D_ENGINEERING_MIM_LF'));
ENDSEC;

DATA;
  /* Actual data entities */
  #10=PRODUCT('Part1','Part1','',(#20));
  #20=PRODUCT_CONTEXT('',#30,'');
  ...
ENDSEC;

END-ISO-10303-21;

2. Syntax Overview

Each line in the DATA section represents an entity instance and follows this standard format:

#ID = ENTITY_NAME(Attribute1, Attribute2, ...);

Symbol	Name	Meaning
#	Hash/Pound	Prefix indicating an Instance ID follows.
10	Instance ID	A unique integer identifier for this object within this specific file.
=	Assignment	Links the ID on the left to the data definition on the right.
ENTITY	Entity Name	The type of data (defined in the EXPRESS schema, e.g., PRODUCT).
(...)	Attributes	The data values or references to other IDs (e.g., (#20)).
;	Semicolon	Mandatory terminator for every STEP statement.

3. Why Instance IDs (#10, #20...) Matter

• Reference System: STEP uses a pointer-based structure. When one entity needs to use another, it refers to its #ID.
• Volatile Nature: These numbers only need to be unique within the file. It is normal and expected for these numbers to change when a file is re-saved by CAD software.
• Numbering Convention: You will often see IDs incremented by 10 (#10, #20, #30). This is a legacy practice to allow for manual insertion of entities (e.g., #15) without renumbering the whole file.

How Instance IDs Reference Each Other

The following diagram shows how entities reference each other using Instance IDs:

Key Points:

• Each arrow shows a reference from one entity to another using #ID.
• #10 references #20 via the frame_of_reference attribute.
• This creates a chain of references that connects product information to geometry.

Forward References Are Allowed

STEP files allow forward references, meaning an entity can reference another entity that is defined later in the file:

Why This Matters:

• Parsers must use two-pass processing: First pass builds an instance map, second pass resolves references.
• You cannot resolve references in a single pass because forward references exist.
• This is why using a hash map (dictionary) to store all instances before resolving references is essential.

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📚 Next Steps

• Data Model Map - Understand the entity hierarchy.

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