ISO 10303-21

Due to its ASCII structure, a STEP-file is easy to read, with typically one instance per line.

The format of a STEP-file is defined in ISO 10303-21 Clear Text Encoding of the Exchange Structure.

The Industry Foundation Classes and earlier CIMSteel Integration Standard (CIS/2) define an EXPRESS schema for building information modeling data and specify ISO 10303-21 as an exchange encoding.

This graph shows the entity instances and the references between them in a file representing a solid model of a cylinder.

On the other hand, its format, and specially the EXPRESS data modelling language has a few disadvantages:[opinion]