R4
FHIR_REF

string

primitive type

Base StructureDefinition for string Type: A sequence of Unicode characters

Official spec
PathCard.TypeDescription & Constraints
string0..*—Primitive Type string
.id0..1Stringxml:id (or equivalent in JSON)
.extension0..*ExtensionAdditional content defined by implementations
.value0..1StringPrimitive value for string