Define Application Messages and User Data

Data is inherited per message ID.

For example, assume a message ID has a command definition for the instance, and the same message ID is defined for a class with reply data. The command and reply data is not merged on the instance, and the class definitions are not inherited at all. Message overrides (OVR) are not inherited at all. All OVRs are used to generate AT entries at the level where they are specified.