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

A code that indicates that a particular type characteristic and its associated element definition aspects apply to this type.

element_id	types
Extension
Extension.extension
Extension.url
Extension.value[x]	code
Extension
Extension.id	http://hl7.org/fhirpath/System.String
Extension.extension	Extension
Extension.url	http://hl7.org/fhirpath/System.String
Extension.value[x]	code