The HasAddIn ReferenceType is a concrete ReferenceType and can be used directly. It is a subtype of the HasComponent ReferenceType.
The semantic of this ReferenceType is to bind an AddIn to another Node that is the source of this Reference. The AddIn concept is described in 4.10.3.
The SourceNode of this ReferenceType shall be an Object or ObjectType. The TargetNode shall be an Object.
One SourceNode can refer to multiple AddIns.