David,
I wasn't around the TC when this was added to the specification but it
appears relatively clear. The toPartyMSH encompasses the default actor and
also covers any other MSH legally able to respond on behalf of the To Party.
Good examples come from previous discussions of multi-hop ebXML-msh use
cases such as store-and-forward MSH instances in an enterprise DMZ. From a
legal and technical perspective, it's perfectly fine for that MSH to
acknowledge a received message.
In my "first half" comments, I mentioned this was not completely apparent
from the text of our specification and recommended a few more words
describing this actor. I also recommended requiring this actor value be
used only in the AckRequested and Acknowledgment elements. According to the
rules we've defined, only the final destination MSH may eliminate
duplicates, re-order messages, et cetera.
Separately, I mentioned in various discussions about acknowledgments it
might be useful to allow the default actor to be named in these two
elements. On second thought, I'm not so sure because the default actor is
already required to act in the fashion described for the toPartyMSH. Other
than the words I mentioned above and re-adding use="required" to the
attribute references in the schema, nothing needs to change.
By the way, the actor you named isn't defined. I'm assuming you meant the
default SOAP actor.
thanx,
doug