I don't think an embedded message/partial has any special semantics at all.
I can see one way that they could arise very easily -- an automatic
digestifier, when fed an incomplete series of message/partial message, might
well produce such a thing.
The whole idea of message/partial is that the pieces are tagged in a globally
unique way. This means that they should reassemble regardless of where they
appear in a hierarchy. However, I suspect that most reassembly agents won't
bother to deal with imbedded message/partial objects. I know my code sure
doesn't; you'd have to explode and redeliver the parts to get the thing to
reassemble.
Ned