ISO 10303-21 - Wikipedia
https://en.wikipedia.org/wiki/ISO_10303-21STEP-File is a widely used data exchange form of STEP. ISO 10303 can represent 3D objects in Computer-aided design (CAD) and related information. 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. ISO 10303-21 defines the encoding mechanism for representing data conforming to a particular …