A correlation ID (and also the request ID) is usually put in the header of a message rather than in the body. The ID is not part of the command or data the requestor is trying to communicate to the replier. In fact, the replier does not really use the ID at all; it just saves the ID from the request and adds it to the reply for the requestor’s benefit. Since the message body is the content being transmitted between the two systems, and the ID is not part of that, the ID goes in the header.
