Section 5.2.3.3. of [EBMS3CORE] is clear that an ebMS3 Receipt MUST have a RefToMessageId referring to the MessageId of the received message.

For no obvious reason, section 5.1.8 of AS4 repeats this in (a) for reception awareness receipts:

"The eb:RefToMessageId in the eb:MessageInfo group in the eb:SignalMessage contains the message identifier of the received message."

This is redundant, and an implementer complains that this sentence has no MUST/ SHOULD/ MAY and therefore is poor specification language.

Section 5.1.8 of AS4 does not repeat this in (b) for non-repudiation receipts, further confusing the developer.

The non-normative XSLT in Appendix B of AS4 generates a RefToMessageId for all receipts, whether reception awareness or NRR receipts.