The three you will be offered
- AP203 — Configuration controlled 3D design
- The original mainstream protocol, aimed at aerospace and defence. Geometry, assembly structure and configuration data. In its first edition it carried no colour; the second edition added colour and a few other presentation items. It is the most conservative choice and the most universally readable.
- AP214 — Core data for automotive mechanical design
- Written for automotive, and in practice the one to pick when you want colour and layers to survive. It carries the same geometry as AP203 plus presentation, and it is very widely supported.
- AP242 — Managed model based 3D engineering
- The current protocol, and the intended successor to both. It merges AP203 and AP214 and adds product manufacturing information: geometric dimensioning and tolerancing, annotation, kinematics and richer assembly data. This is where the standard is going.
The geometry is the same in all three. A plane is a plane, a cylinder is a cylinder, a NURBS surface is a NURBS surface. What changes is the metadata travelling alongside it.
Which one to pick
| Situation | Export as | Because |
|---|---|---|
| You just need the shape somewhere else | AP214 or AP242 | Both are read everywhere current. Either is fine. |
| Colour or layers must survive | AP214 or AP242 | AP203 edition 1 has no colour at all, and you rarely know which edition your exporter writes. |
| You are sending GD&T with the model | AP242 | It is the only one of the three that carries tolerances and annotation as data rather than as a separate drawing. |
| The recipient runs old software | AP203 | Maximum compatibility. Nothing fails to read AP203. |
| A customer specified a protocol | What they specified | Aerospace and defence supply chains often mandate one. Do not improvise. |
| You have no idea | AP214 | The safest default in 2026 for general mechanical work. |
That last row is the honest headline. For the overwhelming majority of exchanges — send a shape to a shop, open a part in another CAD system, get a quote — the protocol choice has no observable effect. The file opens, the geometry is exact, and nobody notices which number was in the dialog.
Where it does bite
- Colour disappearing
- The classic AP203 symptom: a colourful assembly arrives uniformly grey. Re-export as AP214 or AP242 and it comes back.
- Assembly structure flattening
- Usually an exporter setting rather than a protocol limit, but if a multi-part assembly arrives as one body, check both the protocol and the "export as assembly" option.
- PMI silently dropped
- If you carefully applied tolerances in your model and exported AP203 or AP214, they are simply not in the file. Only AP242 carries them, and the receiving system also has to read them.
- A .stp that will not open
- Almost never a protocol problem. Far more often the file is truncated, was written by a broken exporter, or is actually a different format with a renamed extension.
Extensions, and one common confusion
.step and .stp are the same thing. The three-letter form is a legacy of systems that only supported three-character extensions, and every tool treats them identically. You may also meet .stpZ, which is a compressed STEP file, and .stpX, which is an XML encoding of the same data.
A STEP file is plain text. Opening one in a text editor shows a header naming the protocol, which is the quickest way to check what you were actually sent — look for a FILE_SCHEMA line saying something like AUTOMOTIVE_DESIGN, which is AP214, or CONFIG_CONTROL_DESIGN, which is AP203.
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