Confusing description of `trust_anchor` parameter

[cicnavi]

trust_anchor: (RECOMMENDED) The Trust Anchor that the collection endpoint MUST use when collecting Entities. The value is an Entity Identifier. If omitted, the responder sets this parameter to its own Entity Identifier. If the responder does not have a defined Entity Identifier, it MUST use the HTTP status code 400 and set the content type to application/json, with the error code invalid_request.

I don't quite understand what "If the responder does not have a defined Entity Identifier" means... So, I guess the responder doesn't have to be part of the federation (every entity in federation has to have entity ID), or the responder is not actually a trust anchor (or intermediate) in a federation...

Maybe consider clarifying this...

[zachmann]

Yes the idea here was that the responder does not necessarily be part of the / any federation. One can build a perfectly fine entity collection endpoint with just publicly available information; that does not require to be part of a federation. So the responder could be outside and not have an entity id. In that case the trust_anchor parameter would be required.

The responder does not have to be a trust authority to implement the endpoint; however in that case omitting the trust_anchor parameter would by definition always result in an empty response; so also in that case the parameter would be required (but not REQUIRED).

[cicnavi]

I would then suggest to somewhere stress the fact that responder doesn't have to be part of the federation, maybe in trust_anchor parameter description and / or in 3. Entity Collection Endpoint paragraph, which kind of implies this.