"A CDA document is comprised of a header and a body. The header identifies and classifies the document; provides information on authentication, the encounter, the patient, and the provider; and sets the context for the document as a whole. The body contains the clinical report, and is conceptually divided up into nested sections, each containing a narrative block to be rendered along with structured entries and external references." - HL7 CDA, ClinicalDocument "A set of healthcare-related information that is assembled together into a single logical package that provides a single coherent statement of meaning, establishes its own context and that has clinical attestation with regard to who is making the statement. A Composition defines the structure and narrative content necessary for a document. However, a Composition alone does not constitute a document. Rather, the Composition must be the first entry in a Bundle where Bundle.type=document, and any other resources referenced from Composition must be included as subsequent entries in the Bundle (for example Patient, Practitioner, Encounter, etc.)." - HL7 FHIR, Composition