@scott whichever one declares the requisite properties (owned by the forum or some mods on it, scoped to an audience, contains "post" objects bound to a context, etc.) -- if there are any missing properties that you consider "required", then you drop it from consideration.
for example, maybe you consider the "thread" as something special that needs to be declared, e.g. as a ConversationContainer, but i think that's needlessly limiting and not backwards-compatible with unaware implementations.