description: Used with inter-version extensions where the element being referenced by inter-version extension is of type 'canonical' and includes a reference to a resource whose canonical URL is different in the referencing version than it is in the FHIR version where the element was defined.  E.g. if an R5 implementation were using inter-version extensions to support an element that referenced Bar, when in R7, the url would have been .../Foo.  In this situation, the canonical element would have no value and would instead have an extension that referred to the canonical URL of the '../Bar' resource (which would technically not be supported in R7, but is appropriate in R5).
package_name: hl7.fhir.uv.extensions.r4
derivation: constraint
name: AlternateCanonical
type: Extension
elements:
  extension: {index: 0}
  url:
    pattern: {type: Uri, value: 'http://hl7.org/fhir/StructureDefinition/alternate-canonical'}
    index: 1
  value:
    choices: [valueCanonical]
    index: 3
  valueCanonical: {type: canonical, choiceOf: value, index: 4}
package_version: 5.1.0
extensions: {}
class: extension
kind: complex-type
url: http://hl7.org/fhir/StructureDefinition/alternate-canonical
base: http://hl7.org/fhir/StructureDefinition/Extension
version: 5.1.0
required: [valueCanonical, value]