This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I think "endpoints" is not a proper term. It should use intermediates instead. These intermediates act just like network routers, forwarding the message till its final destnation.
The mustUnderstand attribute demands the message receiver have to know the meaning of the element and how to respond. Otherwise, stop processing the message further, thow an exception back to the originator.