identify books. The URN is also nominated as a "public (unique) ID". In that kind of public use, the need for a central authority las a unique and necessary URN-resolver is a problem. In that context, the identifier's user must query the authority about the correct ID from some object's metadata, like year or title. IDs like ISBN, which need a central authority, are also named "opaque IDs".

A common use of Lex URNs is to express "transparent identifiers", which can be built up by simple