A typical message header

{
  "resourceType" : "MessageHeader",
  "id" : "1cbdfb97-5859-48a4-8301-d54eab818d68",
  "text" : {
    "status" : "generated",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\">\n\t\t\t<p>Update Person resource for Peter James CHALMERS (Jim), MRN: 12345 (Acme Healthcare)</p>\n\t\t</div>"
  },
  "eventCoding" : {
    "system" : "http://example.org/fhir/message-events",
    "code" : "admin-notify"
  },
  "destination" : [{
    "endpointUrl" : "llp:10.11.12.14:5432",
    "name" : "Acme Message Gateway",
    "receiver" : {
      "reference" : "http://acme.com/ehr/fhir/Practitioner/2323-33-4"
    }
  }],
  "source" : {
    "endpointUrl" : "llp:10.11.12.13:5432",
    "name" : "Acme Central Patient Registry",
    "software" : "FooBar Patient Manager",
    "version" : "3.1.45.AABB",
    "contact" : {
      "system" : "phone",
      "value" : "+1 (555) 123 4567"
    }
  },
  "reason" : {
    "coding" : [{
      "system" : "http://terminology.hl7.org/CodeSystem/message-reasons-encounter",
      "code" : "admit"
    }]
  },
  "response" : {
    "identifier" : {
      "value" : "5015fe84-8e76-4526-89d8-44b322e8d4fb"
    },
    "code" : "ok"
  },
  "focus" : [{
    "reference" : "Patient/example"
  }],
  "definition" : "http:////acme.com/ehr/fhir/messagedefinition/patientrequest"
}