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

The FHIR type of a property - used on the value property of a primitive type (for which there is no type in the FHIR typing system), and Element.id, Resource.id, and Extension.url. All of these have a non-FHIR type in thir structure definition, and this specifies the applicable FHIR type.