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.
NameFlagsCardTypeDescription & Constraints
Extension
0..1Alternative reference (target type is wrong)
 
extension
?..0
 
url
?..?
 
value[x] *
1..?
 
 
valueReference *
1..?Reference