"The <> being an empty URI reference always refers to the document it is written in." I am guessing a document base URI becomes whatever URI the triples are hosted at. (the absolute URI).
Further exploration of <> went here. It talks about <> referring to a base URI.
5.1.3. Base URI from the Retrieval URI If no base URI is embedded and the representation is not encapsulated within some other entity, then, if a URI was used to retrieve the representation, that URI shall be considered the base URI. Note that if the retrieval was the result of a redirected request, the last URI used (i.e., the URI that resulted in the actual retrieval of the representation) is the base URI."