(URN-like thing =~ "URI", the generic identifier)
The HTTP spec has, in the object metainfo, a URI: header.
It specifies a URI for the object.
The "vary=" parameter of this header specifies whether the
URI refers to the exact binary pattern, or whether it can
refer to "variants" of language, content-type or version.
(These are the only variants curerntly explicit, one can
imagine others)
Caching systems obviously use this information intelligently
to know whether they have a copy verbatim of an object or a copy of
one instance of a generic object.
Tim