http://hl7.org/fhir/StructureDefinition/operationdefinition-allowed-type|5.2.0

When the base type is an abstract type (e.g. Resource or Element) then this extension defines which concrete types are allowed to be used for a parameter. In the absence of this extension, any type is allowed. Replaced by OperationDefinition.parameter.allowedType in R5+
NameFlagsCardTypeDescription & Constraints
Extension
0..*Which types are allowed for a parameter, when the base type is Abstract
 
extension
?..0
 
url
?..?
 
value[x] *
1..?
 
 
valueUri *
1..?uri