An incomplete list of parts making up STEP (ISO 10303): The 'APs' utilize the lower-level information of integrated resources in well defined combinations and configurations to represent a particular data model of an engineering or technical application.
An ATS is a formal description on how to test STEP implementations for conformance.
The original plan of STEP was to have for every AP 2xx a corresponding ATS 3xx, but only a few were finally realized till today.
AICs are specializations of the integrated application and generic resources.
AICs did not exist back in 1994 when the first release of STEP got published.