File formats
STEP
Standard for the Exchange of Product Model Data
Definition
STEP is the ISO 10303 family of standards for exchanging 3D product data between CAD systems, usually saved as .step or .stp files. A STEP file carries exact B-Rep solid geometry that another CAD program can measure and edit, but it normally does not carry the original feature history.
Also known as: STP, ISO 10303, STEP file, .step
Updated
What a STEP file contains#
STEP is not one format but a family of ISO 10303 parts. The STEP Tools overview describes it as a comprehensive ISO standard for representing and exchanging digital product information. Most CAD users meet it as a Part 21 text file with a .step or .stp extension.
The useful part for mechanical work is precise B-Rep geometry: faces, edges, and vertices defined by exact surfaces and curves. A cylinder in a STEP file is a cylinder with a radius, not a ring of triangles. The receiving CAD program can select faces, measure diameters, add fillets, and use the geometry for CAM or drawings.
Application protocols#
Different industries use different subsets of STEP, called application protocols (APs). The ones CAD users see most are AP203, AP214, and AP242. The CAx Interoperability Forum tests AP242, "Managed Model-based 3D Engineering", which covers precise B-Rep and tessellated geometry, product and manufacturing information (PMI), kinematics, composites, and validation properties.
The export option you pick matters. An AP203 or AP214 export may carry geometry and assembly structure only, while an AP242 export can also carry semantic PMI when both the sending and receiving software support it.
What STEP usually loses#
A STEP file describes the result of modeling, not the steps that produced it. The feature tree, sketch constraints, driving dimensions, and design intent stay in the native CAD file. After import, most CAD programs treat the geometry as a single dumb solid that can be edited with direct modeling tools or used as a base for new features.
STEP was created to replace a set of older national exchange formats. The STEP Tools history lists SET in France, VDAFS in Germany, and IGES in the USA as the predecessors that the ISO effort set out to unify.
Why it matters for text-to-CAD#
STEP is the most practical test of whether an AI tool produced engineering geometry. If the output opens as a valid solid in Fusion, SolidWorks, Onshape, or FreeCAD, it can be inspected and developed further. If a tool only exports STL, OBJ, or glTF, the output is a mesh and the faces, edges, and exact dimensions are gone.
Sources
Related terms
B-Rep
B-Rep is the way professional CAD kernels describe a solid: by its boundary, made of exact surfaces (faces), the curves where they meet (edges), and the points where edges end (vertices). Because the geometry is exact and the topology is explicit, B-Rep models can be measured, filleted, dimensioned, and manufactured precisely.
IGES
IGES is an older neutral format for exchanging CAD geometry, developed in the United States. It still appears in legacy workflows and for surface data, but STEP has largely replaced it for solid models.
PMI
PMI is the dimensions, tolerances, GD&T, materials, surface finishes, and notes attached to a 3D CAD model instead of, or in addition to, a 2D drawing. It is the core content of model-based definition.
STL
STL is a simple mesh file format that stores a surface as a list of triangles, each with a normal and three vertices. It is the long-standing default for 3D printing, but it has no units, colors, or exact geometry.