http://hl7.org/fhir/StructureDefinition/alternate-canonical|5.2.0

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).