They're referring to multipart-MIME - that's what ebXML is using to assemble different documents (of any format) into one message.
The ebXML TRP spec is online, btw - check out the ebXML website (locatable via google search).
This discussion is probably more appropriate for the xml-dist-app list.
Eve -- Eve Maler +1 781 442 3190 Sun Microsystems XML Technology Center eve.maler @ east.sun.com