I think your question is, should SequenceNumber be incremented at one end or at both ends? SequenceNumber should increment in only one (each) direction. Our intent was to assure that messages sent from one business partner arrive in the right order rather than to sequence an entire conversation, but I don't see this clearly spelled out. We may need to add a clarifying statement in the specification. Looking at the specification, it appears to be confusing. If SequenceNumber is incremented per conversation which means in both directions since a conversation goes in both directions, there is a problem. Suppose CompanyA sends to CompanyB with sequence number 1. Then CompanyA sends another message to CompanyB with sequence number 2 at the same time that CompanyB sends a reply to CompanyA with sequence number 2. We have two messages with the same sequence number. If the sequence is per-conversation, which 2 is correct? Am I understanding the question correctly? I am copying the ebXML-Messaging list to solicit other comments. Regards, David Fischer Drummond Group.