> If you run nntp.example.com and someone else runs nntp.example.org, and you both create the local group example.talk.general there will be no "collision", there will simply be same group on two different servers.
That is correct, but the newsgroups might not be connected, and that can cause confusions. If they are connected then it is not a problem of course, but some servers might not be connected.
> If the issue is you don't want posts from other servers on your server's local copy of example.talk.general, or vice versa, then you simply don't send or receive articles for example.talk.general to or from other servers.
While that is possible, some servers may copy it if you do not want to, and you may have a description or purpose which does not match that on the other server, and it can also cause confusion in the client; you may be using multiple servers, each with a newsgroup called example.talk.general, and you will not tell the difference so easily. This might not be a problem for you, but sometimes these issues are significant. Later they might want to copy messages between the two servers, but what if they don't want that newsgroup? It will cause many confusions.
> So what happens if someone creates the Usenet group un2.org.example.a.b.c which you already have as an "Unusenet" group?
If someone wants such a newsgroup, they can connect it to the example.org server, so that messages will be propagated. Otherwise, you will have distinct local newsgroups with the same name, and there can be confusions as noted above.
Ordinary Usenet newsgroup names do not start with "un" and a number, so such things are not expected to occur unless someone deliberately tries to cause collisions (for which the same thing can happen with UUIDs, as well as URIs to identify namespaces in XML and RDF). (By definition, "un2.org.example.a.b.c" is a "Unusenet" group. If someone creates it on Usenet, they should know to connect it with the example.org server.)
Nobody is required to use this Unusenet specification. If it doesn't help, don't use it. But, it is there if you need it.
If you are creating newsgroups in existing hierarchies according to their policies for doing so, then Unusenet is not helpful and can easily be avoided.
(But, see, for example, the thread starting at on news.software.nntp for some other points of view. They mention creating their own hierarchy, to avoid problems with adding new newsgroups to existing ones; Unusenet is merely one possible way to do so without collision, should you need or want such a thing.)
> ... "Unusenet" is simply a word you've come up with to describe a standard, fundamental feature of Usenet: creating (local) newsgroups.
Well, not quite. The word "Unusenet" I made up refers to the specific set of namespaces which are named according to the specifications in that document. The fundamental feature of creating newsgroups (local or otherwise) is independent, and can be created with or without Unusenet namespacing.