R4
FHIR_REF

Reference

complex type

Base StructureDefinition for Reference Type: A reference from one resource to another.

Official spec
PathCard.TypeDescription & Constraints
Reference0..*—A reference from one resource to another
.id0..1StringUnique id for inter-element referencing
.extension0..*ExtensionAdditional content defined by implementations
.referenceΣ0..1stringLiteral reference, Relative, internal or absolute URL
.typeΣ0..1uriType the reference refers to (e.g. "Patient")
Bindingresource-types(extensible)
.identifierΣ0..1IdentifierLogical reference, when literal reference is not known
.displayΣ0..1stringText alternative for the resource