ChannelMessageType:
    | "offer"
    | "counter"
    | "accept"
    | "reject"
    | "sealed-envelope-commit"
    | "sealed-envelope-reveal"
    | "abort"

Allowed message types — verbatim from DACS-3 §8.3.3. Closed union by design: the substrate enforces the conformance set; application bodies are carried in body: unknown.