I agree that "OCP application" is awkward. However, we are not talking just about OCP message payload. The term refers to the set of agreements (a protocol) covering things like: - meta-data encoding - required meta-data fields and their meaning - data encoding - data meaning The exact combination is unknown a priory. For example, in some cases, it may make sense to agree on meta-data and let meta-data describe data encoding and meaning.
The "other side" of OCP communication does not know what it is getting in meta-data and data fields. It needs to be told about encoding and meanings, just like, say, HTTP defines HTTP message encoding and meaning.
Again, a better term than "OCP application" would be great, but it has to cover all of the above things. It is the result of OCP agents negotiations about data and meta-data.