ISO documents have a very formal structure which you should become familiar with if you want to read the spec.
The EXPRESS definitions for the Application Interpreted Model (AIM) describe data structures found in an exchange file. The version published with the document is wg3n2097_ap238.exp
All recent work is using a newer schema updated with definitions from STEP CAD AP242. This adds improved GD&T definitions, tesselated models, and better kinematics.
The first edition was published with four conformance classes (CCs). These called out subsets of the model for increasingly complex uses.