The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on. In other words, any concept that might be the target of an author's hypertext reference must fit within the definition of a resource. A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time.
The more that I've learned about web development, the more that I've come to appreciate the thoroughness and thoughtfulness of the authors of the HTTP RFC and Roy Fielding's dissertation. It seems like the answers to most problems come down to "There's a section of the spec for that.