extension_ property
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 manageable, 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;