The phases are quite separate, and each phase allows for considerable variation. The search service is used to communicate the selection criteria. An RPN Query specifies a set of constraints to be met by interesting items. A current project is developing a Z39.50 interface to Conquest s semantic- net based dictionary, so there isn t a protocol issue involved with the set of categories to be used in GILS records. This is a significant concern, but not related to the choice of protocol.
URNs are an IETF development, currently underway.
One might think of them as intending to be dynamic hyperlinks. Z39.50 will carry them, however they are eventually defined. It describes a pretty big protocol with lots of options. It really isn t hard if you take things one at a time. It isn t a protocol.