David:
One reason why the AckRequested element MUST be in the message is that it
may be targeted for the next MSH, if intermediate Acks rather than
end-to-end Acks are desired. Even though the from party and to party may be
aware of the CPA parameters, intermediaries are not privy to that
information.
For consistency reason, the QualityOfServiceInfo element with the
duplicateElimination attribute set to true should also be present in the
message header to trigger duplicate elimination behavior on the part of the
receiver.
Regards,
-Arvola