We would add a MessageDefinition resource for each message type. MessageHeader would point to the MessageDefinition resource. MessageDefinition resource .focus.profile element would point to the desired structure definition to be expected in the message. On the US PH Reporting Bundle, we would have 3 slices. First slice - Message Header which is mandatory. Second Slice - optional Content Bundle. Third Slice - Any Resource. Content IGs can constrain the US PH Reporting Bundle to only limit to a specific type of structure such as a Content Bundle or a specific Resource. This should also be specified in the MessageDefinition resource pointed to within the MessageHeader resource.  MedMorph uses FHIR Messaging which contains the usage of a MessageHeader within a top level reporting bundle. The content of the message can be expressed as a Content Bundle which can be of type Document to support use cases that require Composition. The Content Bundle can also be just a list of all the resources. In addition since the data is transmitted via intermediaries, who may provide content services such as validation the inner bundle provides a way to extract the data easily and perform the necessary validations.