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FHIR resources contain the following items:
A resource can usually reference other resources.
For example, a patient resource can reference an allergy resource. However, resources are independent; you don’t need other resources to correctly interpret a resource. Resources reference each other extensively to form a web of information. You need to resolve references to fully understand the data. A reference is relative to a server-based URL.
<Procedure xmlns="http://hl7.org/fhir">
<subject>
<reference value="Patient/23"/>
</subject>
All resources carry an HTML representation of the content to ensure clinical safety in the case that the receiver does not understand the content.