STEP file format is defined by ISO 10303-21 Clear Text Encoding of the Exchange Structure. There are three editions of ISO 10303-21. First edition was released in 1994 and last was released in 2016. In last edition added is anchor, reference and signature sections to support external references, support for compressed exchange structures in an archive, digital signatures and UTF-8 character encoding.

ISO 10303-21 structure it contains two sections, header and data section. Header section has fixed structure consisting 3 to 6 groups in given order. The DATA section contains application data according to one specific express schema encoded following some simple principles.