MessageHeader on the FHIR specification website.

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.

The body should be a list of valid JSON patches for MessageHeader. For example, to update a Slot status to busy, this would be the body:

[
  {
    op: 'replace',
    path: '/status',
    value: 'busy',
  },
],