A resource can map to the empty set, which allows references to be made to aconcept before any realization of that concept exists -- a notion that was foreign to most hypertext systems prior to the Web.
This is an interesting concept. Through the use of status codes, I can indicate to a client the state of a URI. I can issue URIs to resources that do not exist yet and if a client dereferences the URI an appropriate status code indicates that it does not yet exist. The client can continue to request until it either receives a different status code or the representation is returned.