Must (Try) to Understand This?

Sean McGrath writes about "must understand" rules in loosely coupled message passing...

Whatever about the pros and cons of REST versus SOAP, I think it is abundantly clear that the mustUnderstand model [1] is a key concept in developing loosely coupled systems that can evolve independently.

I need April 2 asap.

One thought this does provoke though, for real, is that a sender probably has no business telling a receiver what it must understand or to what degree. I'm all for ways for receivers to describe, more or less formally, what they will understand and how they will understand it. Senders just send messages and set their expectations as best as possible. Hopefully both senders and receivers have robust fallback capabilities.