R4
FHIR_REF

uuid

primitive type

Base StructureDefinition for uuid type: A UUID, represented as a URI

Official spec
PathCard.TypeDescription & Constraints
uuid0..*—Primitive Type uuid
.id0..1Stringxml:id (or equivalent in JSON)
.extension0..*ExtensionAdditional content defined by implementations
.value0..1StringPrimitive value for uuid