1. A protocol for conveying data during an e-commerce session with a polymorphic response, comprising: initiating a session with a first message from a buyer application to a broker application and a second message from the broker application to a supplier application; conducting the session between the buyer application and the supplier application; and responding during the session with an additional message including: a schema identifier for the additional message, resolvable in a context of a system identifier; and a polymorphic response comprising a type and a version, wherein the polymorphic response including additional data elements corresponding to values assigned to the type and version.