R4
FHIR_REF

base64Binary

primitive type

Base StructureDefinition for base64Binary Type: A stream of bytes

Official spec
PathCard.TypeDescription & Constraints
base64Binary0..*—Primitive Type base64Binary
.id0..1Stringxml:id (or equivalent in JSON)
.extension0..*ExtensionAdditional content defined by implementations
.value0..1StringPrimitive value for base64Binary