 step-file is the most 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 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 10303-21:2002 industrial automation systems and integration -- product data representation and exchange -- part 21: implementation methods: 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. a step-file is 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.iso tc184/sc4 secretary "cumulative list of resolutions" resolution 583 (stuttgart, germany, - june 2003) "registration of sc4 mime-types",  == history == some details to take care of: * the first edition of iso 10303-21:1994 had some bugs, which were corrected by a technical corrigendum. therefore, it is recommended that users study the second edition instead (see below). * the second edition iso 10303-21:2002, including all the fixes and extensions for several data sections. * part 21 defined two conformance classes. they differ only in how to encode complex entity instances. ** conformance class 1 is always used enforce the so-called internal mapping, which is more compact. ** conformance class 2, which is not used in practice, always enforces the external mapping. in theory this would allow better ap interoperability, since a postprocessor may know how to handle some supertypes, but may not know the specified subtypes. * the 1st edition of part 21 enforces the use of so-called short names, which are optional in the 2nd edition. in practice, however, short names are rarely used. * the 2nd edition allows multiple data sections to be used. in practice, however, most implementations only use a single data section (1st edition encoding). == iso 10303-21 building blocks == === example === a typical example looks like this: iso-10303-21; header; file_description( /* description */ ('a minimal ap214 example with a single part'), /* implementation
ISO_10303