The K8s GW API has precedent for using typed references to "Virtual" Resources (i.e. non-existent) as a way of providing parameters (e.g. to a filter on a route) without needing to define additional CRD types. This is done in Istio as a way of enabling users to specify backend refs to hostnames rather than directly to a ServiceEntry object.
Because these reference types are effectivey a part of the application API without being easy to document, this package is separated and commented for easier integration into documentation.