The header for a message exchange that is either requesting or responding to an action. The reference(s) that are the subject of the action as well as other information related to the action are typically transmitted in a bundle in which the MessageHeader resource instance is the first resource in the bundle. To enable sending system insert its own identifiers into API submitted eReferral, and have that returned with subsequent messages, an extension is created on the MessageHeader.identifier element for referral id.