Structure of Application Protocols
AP-238 describes the STEP-NC application interpreted model (AIM) for the application reference model (ARM) defined by ISO 14649-10, 11, 12, 111 and 121, augmented with ISO 10303 product geometry, geometric dimensioning and tolerancing, and product data management information. The AIM is for implementations, and the ARM is for understanding the meaning. This two-level approach lets the folks who make the ARM document the information requirements clearly, while the actual data is represented in an extensible way that remains stable across APs and handles aspects of a product never considered by the folks who made the ARM.
In the AP document, clause 4.1 defines the ARM "units of functionality" which are groups of information requirements described as application objects. The application objects describing the machining process are all referenced from the ISO 14649 parts while the STEP-specific things like GD&T and PDM are defined in clause 4.2. Features are defined by both, so we refer to the 14649 definitions and note the few cases in clause 4.2 where we need to add some information for harmonization purposes.
The AIM is defined by the mapping tables in clause 5. As you look through these tables, you will see reference paths that show how ARM concepts are represented by a collection of AIM objects. So you will refer to the ARM descriptions to understand what is in the AP, and the AIM descriptions to understand how it is represented and exchanged by implementations.
The short and long form EXPRESS schemas for the AIM give you the structures used by the mapping tables. The short form schema only contains the new subtypes of existing integrated resources. APs are allowed to create subtypes for clarity, but they are not allowed to add any new attributes or create new top-level entities. The long form EXPRESS contains all of the definitions required to work with the AP-238 AIM.