http://hl7.org/fhir/StructureDefinition/diagnosticReport-focus|5.2.0

States what the diagnostic report is about if it is not the subject of the report

element_id	types
Extension
Extension.extension
Extension.url
Extension.value[x]	Reference
Extension
Extension.id	http://hl7.org/fhirpath/System.String
Extension.extension	Extension
Extension.url	http://hl7.org/fhirpath/System.String
Extension.value[x]	Reference