http://hl7.org/fhir/StructureDefinition/elementdefinition-defaulttype|5.2.0

The default type that applies if the instance doesn't specify a particular type, and there is a choice of types. Only for use on logical models.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/canonical
Extension.value[x].targetProfilehttp://hl7.org/fhir/StructureDefinition/StructureDefinition