Glossary

IMPORTANT

We strongly recommend reading this page first.

Understanding the STEP standard begins with its terminology. By grasping these "Top 5 Most Important Terms," understanding other documents will become significantly easier.

⭐ Top 5 Most Important Terms (Start Here)

The five core concepts that every implementer should understand first.

1. AP (Application Protocol) ★★★

A subset of the standard defined for a specific industry or application.

AP Evolution:

What Implementers Need to Know:

AP203, AP214, and AP242 are the major versions.
Different APs support different entities (data structures).
You can identify the AP in the FILE_SCHEMA of the STEP file's HEADER section.

Example within a STEP file:

step

FILE_SCHEMA(('AP242_MANAGED_MODEL_BASED_3D_ENGINEERING_MIM_LF { 1 0 10303 442 1 1 4 }'));

→ This file uses AP242.

Related Terms: MIM, AIM

2. Entity ★★★

The building blocks of STEP data. Equivalent to a "Class" in Object-Oriented Programming.

Relationship Concept:

What Implementers Need to Know:

Written in all UPPERCASE (e.g., PRODUCT, SHAPE_REPRESENTATION).
Has attributes.
Supports inheritance (SUPERTYPE/SUBTYPE).

Example within a STEP file:

step

#10 = PRODUCT('Part_A','Part_A','description',(#20));

PRODUCT is the entity name.
#10 is the Instance ID (a unique identifier within this file).
The values inside parentheses are the attribute values.

Implementation Note:

Entity names are technically case-insensitive per the specification, but they are typically written in uppercase.
Parsers must correctly resolve references using the #number format.

Related Terms: EXPRESS, Instance

3. B-rep (Boundary Representation) ★★★

A method of defining shapes by their boundaries: faces, edges, and vertices.

Difference from STL:

	B-rep (STEP)	STL
Representation	Mathematically precise surfaces (NURBS, etc.)	Triangular mesh (approximate)
Precision	Theoretically infinite	Dependent on mesh size
File Size	Medium to Large	Small to Medium
Editability	Parametrically editable	Difficult

B-rep Hierarchy in STEP:

Detailed B-rep Structure:

The following diagram shows how B-rep elements connect to form a complete solid:

Connection Flow:

Solid → Shell: A solid is bounded by a closed shell
Shell → Faces: A shell contains multiple faces
Face → Edge Loop: A face is bounded by edge loops
Edge Loop → Oriented Edges: An edge loop is a sequence of oriented edges
Oriented Edge → Edge Curve: References the geometric edge
Edge Curve → Vertices: Connects two vertices
Vertex → Point: Contains the actual 3D coordinates

Key Insight: B-rep separates topology (how elements connect) from geometry (the actual shapes). This allows the same topology to represent different geometric shapes.

Related Terms: NURBS, Tessellation

4. PMI (Product and Manufacturing Information) ★★

Non-geometric design intent, such as dimensions, tolerances, and surface finish.

Two Types of PMI:

Graphical PMI (Display type):

Annotations for humans (visual lines and text).
Can be displayed in 3D viewers.
Cannot be semantically interpreted by computers.

Semantic PMI (Representation type):

Computer-processable information.
Represented by entities like GEOMETRIC_TOLERANCE.
Fully supported in AP242.

Importance for Implementers:

The need for PMI dictates the AP choice (PMI required → AP242 mandatory).
Parsing Semantic PMI requires knowledge of Geometric Dimensioning and Tolerancing (GD&T).
PMI compatibility between CAD systems is not always perfect (refer to CAx-IF guidelines).

Related Terms: GD&T, MBD

5. EXPRESS ★★

The data modeling language used to define STEP (ISO 10303-11).

What Implementers Need to Know:

The "Schema" for each AP is written in EXPRESS.
It defines entities, attribute types, and constraints.
Refer to the EXPRESS schema in the standard documents during implementation.

EXPRESS Example:

express

ENTITY product;
  id : identifier;
  name : label;
  description : OPTIONAL text;
  frame_of_reference : SET [1:?] OF product_context;
END_ENTITY;

OPTIONAL: This attribute can be omitted.
SET [1:?]: A set of one or more unique items.

Mapping to Programming Languages:

EXPRESS	Java/C++	Python	Meaning
`ENTITY`	`class`	`class`	Class Definition
`TYPE`	`typedef`	`NewType`	Type Alias
`OPTIONAL`	`Optional<T>`	`Optional[T]`	Nullable
`SET [1:?]`	`Set<T>`	`set`	Unique Set
`LIST [0:?]`	`List<T>`	`list`	Ordered List

Related Terms: Entity, Schema

📂 Glossary by Category

File Structure

Part 21 (ASCII / Clear Text)

The most common text-based file format for STEP, known by extensions .stp or .step (ISO 10303-21).

Structure:

step

ISO-10303-21;
HEADER;
  FILE_DESCRIPTION(...);
  FILE_NAME(...);
  FILE_SCHEMA(...);
ENDSEC;
DATA;
  #10 = PRODUCT(...);
  #20 = ...;
ENDSEC;
END-ISO-10303-21;

Part 28 (XML)

An XML representation of STEP data. More redundant than Part 21 but compatible with XML technologies.

Instance

Concrete data with specific values for an entity. In #10 = PRODUCT(...), #10 is the instance identifier.

Implementation Note:

Instance IDs start at 1 and are unique within the file.
Forward references (referring to an ID defined later in the file) are permitted.

Geometry & Topology

NURBS (Non-Uniform Rational B-Spline)

A mathematical way to represent complex curves and surfaces. Extensively used in STEP B-rep.

PCurve

A 2D curve defined in the parameter space (UV) of a surface. Critical for mapping topology to geometry accurately.

Knot Vector

A sequence of parameter values that control the distribution of a spline. Used in NURBS to define segment boundaries and continuity.

G1/G2 Continuity

Measures of smoothness between geometric elements. G1 (Tangential) ensures no sharp creases; G2 (Curvature) ensures perfectly smooth transitions for reflections.

Manifold

A geometric property where every edge is shared by exactly two faces. "Non-manifold" geometry (e.g., three faces meeting at one edge) is harder for CAD systems to process.

CSG (Constructive Solid Geometry)

A method of building complex shapes using Boolean operations (Union, Subtract, Intersect) on primitives like blocks and spheres. STEP supports both CSG and B-rep.

Tessellation

Approximating a surface with a mesh of triangles or polygons. Standardized in AP242.

Uses:

Fast visualization (polygons are lighter to render).
File size reduction (using tessellation alongside or instead of B-rep).

ADVANCED_FACE

The most common entity for representing faces in STEP. Defines the face boundary (edge loops) and its surface geometry.

Management Data

PRODUCT

A top-level entity representing the part itself.

Attributes:

id: Part ID (string).
name: Part name.
description: Description (optional).
frame_of_reference: Context.

PRODUCT_DEFINITION

A definition of a product in a specific context (design, analysis, manufacturing). Geometry data is linked to this entity.

PRODUCT_DEFINITION_SHAPE

The "bridge" between management data and geometry. Links PRODUCT_DEFINITION to a SHAPE_REPRESENTATION.

PMI & Tolerancing

GD&T (Geometric Dimensioning and Tolerancing)

A system for defining and communicating engineering tolerances.

Major Tolerance Types:

Flatness, Straightness, Circularity (Form)
Position, Coaxiality, Symmetry (Location)
Parallelism, Perpendicularity, Angularity (Orientation)

GEOMETRIC_TOLERANCE

The base type for entities representing GD&T in STEP. Subtypes include POSITION_TOLERANCE, FLATNESS_TOLERANCE, etc.

DATUM

A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A basis for tolerances.

AP & Schema

MIM (Mapped Interpreted Model)

The data model implementers actually work with, written in EXPRESS (the components of a STEP file).

AIM (Application Interpreted Model)

A shared data model across APs that serves as the foundation for the MIM.

ARM (Application Reference Model)

A high-level information model from a user's perspective, used to define business requirements. It is mapped to the MIM.

Schema

The entire data model definition written in EXPRESS. Each AP has its own unique schema.

MBD & Digital Thread

MBD (Model Based Definition)

A practice of using 3D models (the "Master") to contain all design and manufacturing information.

Difference from Traditional Methods:

Traditional: 2D drawings are the master; 3D is a reference.
MBD: 3D model is the master; 2D drawings are unnecessary or subordinate.

LOTAR (Long Term Archiving and Retrieval)

A standard for the long-term preservation and retrieval of digital data. Supported in AP242.

Purpose:

Ensure data remains readable decades later.
Crucial in aerospace and defense industries.

CAx-IF (CAD-CAx Implementor Forum)

A group that establishes STEP implementation guidelines among CAD vendors.

Importance for Implementers:

Following CAx-IF recommended practices improves interoperability.
Provides test cases and benchmark files.

Official Standards: ISO 10303 series (Paid)
CAx-IF Recommended Practices: https://www.cax-if.org/
EXPRESS Schemas: Found in the annexes of each AP standard document.

📚 Next Steps

Getting Started Guide - Grasp the overall picture of STEP.
Geometry and Topology - Deep dive into mathematical representation.

Back to README

Glossary ​

⭐ Top 5 Most Important Terms (Start Here) ​

1. AP (Application Protocol) ★★★ ​

2. Entity ★★★ ​

3. B-rep (Boundary Representation) ★★★ ​

4. PMI (Product and Manufacturing Information) ★★ ​

5. EXPRESS ★★ ​

📂 Glossary by Category ​

File Structure ​

Part 21 (ASCII / Clear Text) ​

Part 28 (XML) ​

Instance ​

Geometry & Topology ​

NURBS (Non-Uniform Rational B-Spline) ​

PCurve ​

Knot Vector ​

G1/G2 Continuity ​

Manifold ​

CSG (Constructive Solid Geometry) ​

Tessellation ​

ADVANCED_FACE ​

Management Data ​

PRODUCT ​

PRODUCT_DEFINITION ​

PRODUCT_DEFINITION_SHAPE ​

PMI & Tolerancing ​

GD&T (Geometric Dimensioning and Tolerancing) ​

GEOMETRIC_TOLERANCE ​

DATUM ​

AP & Schema ​

MIM (Mapped Interpreted Model) ​

AIM (Application Interpreted Model) ​

ARM (Application Reference Model) ​

Schema ​

MBD & Digital Thread ​

MBD (Model Based Definition) ​

LOTAR (Long Term Archiving and Retrieval) ​

CAx-IF (CAD-CAx Implementor Forum) ​

🔗 Related Resources ​

📚 Next Steps ​

Glossary

⭐ Top 5 Most Important Terms (Start Here)

1. AP (Application Protocol) ★★★

2. Entity ★★★

3. B-rep (Boundary Representation) ★★★

4. PMI (Product and Manufacturing Information) ★★

5. EXPRESS ★★

📂 Glossary by Category

File Structure

Part 21 (ASCII / Clear Text)

Part 28 (XML)

Instance

Geometry & Topology

NURBS (Non-Uniform Rational B-Spline)

PCurve

Knot Vector

G1/G2 Continuity

Manifold

CSG (Constructive Solid Geometry)

Tessellation

ADVANCED_FACE

Management Data

PRODUCT

PRODUCT_DEFINITION

PRODUCT_DEFINITION_SHAPE

PMI & Tolerancing

GD&T (Geometric Dimensioning and Tolerancing)

GEOMETRIC_TOLERANCE

DATUM

AP & Schema

MIM (Mapped Interpreted Model)

AIM (Application Interpreted Model)

ARM (Application Reference Model)

Schema

MBD & Digital Thread

MBD (Model Based Definition)

LOTAR (Long Term Archiving and Retrieval)

CAx-IF (CAD-CAx Implementor Forum)

🔗 Related Resources

📚 Next Steps