ISO 10303-21
STEP-File is the most widely used data exchange form of STEP. Due to its ASCII structure it 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 on how to represent data according to a given EXPRESS schema, but not the EXPRESS schema itself. STEP-File are also called p21-File and STEP Physical File. The file extensions .stp and .step indicates that the file contain data conforming to STEP Application Protocols while the extension .p21 should be used for all other purposes.