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

Used when the target of the reference has a type that is not allowed by the definition of the element. In general, this should only arise when wrangling between versions using cross-version extensions.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/Reference
Extension.value[x].targetProfilehttp://hl7.org/fhir/StructureDefinition/Resource