http://hl7.org/fhir/StructureDefinition/structuredefinition-fhir-type|4.0.1

The formal FHIR type of a property - used for the value property of a primitive type (for which there is no type in the FHIR typing system), and Element.id and Extension.url.
element_idtypes
Extension
Extension.extension
Extension.url
Extension.value[x]
Extension
Extension.id
Extension.extension
Extension.url
Extension.value[x]