extension_ property
extension_ ("extension") May be used to represent additional information that is not part of the basic definition of the resource. To make the use of extensions safe and managable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension.
Implementation
@JsonKey(name: 'extension')
List<FhirExtension>? get extension_ => throw _privateConstructorUsedError;