I don't like this.  We are building something where the next layer (your tier 3) has the final say on the semantics of frames and their ordering.  This undermines that.  There is no need to finagle this with statements like this.  The HTTP usage defines the semantics of this frame in a strict set of cases.  Where the semantics are undefined, they are undefined.

The HTTP usage does need to be clearer in some parts on when different endpoints are able to send what.  That's where I'd prefer to see this addressed.