Yes, it is preferrable to have th emessage exchange specification as one specification then the message definitions as separate documents so that you have separation of concerns and can evolve messages without opening the infrastructure to reballoting with each new message.

Existing Wording:

FHIRCast and CDS Hooks specify their events (or hooks) in separate specifications, using their own maturity models and lifecycles. Should the SMART Web Messaging adopt similar patterns for its message types and activities?