As I recall, the explanation for each forum was at the entry level for each forum and not displayed within the forum itself.
The PL/SQL XML Programming space refers to the Oracle XML Developer Kit. It is maintained by a different team than the XMLDB so the need for a separate space for it makes sense. That said, what it covers is also part General XML and XQuery if you dig into what you can do in the XDK.
Some questions that would qualify for the XML DB space.
Anything related to schema registration or downstream of schema registration. Such as querying/performance/indexes/etc.
Issues parsing XML via using XMLTable (or XQuery, such as the new XQuery update added in 126.96.36.199) (Yes that can conflict)
Storage of data in XMLType columns
I to have always been a bit unclear on the exact division between groups in this XML space. I see the XQuery space as being a subset of the General XML space. I watch all three forums, as they are normally low volume and you never know where a question will pop up. As you have seen, questions often end up in the wrong forum and often General XML gets an influx of questions that should go into the Business Intelligence space as well. There is no good way right now to start a generic post and let the system suggest forums that the post could go into, based on content or software involved. That would be a nice touch for the future.
But you exactly describe the problem. Even for experienced users it is hard to tell where a certain problem should be asked. How is a "normal" user supposed to know the differences? Mostly she/he will not even know where the problem is and why it doesn't work or which technic to apply to the actual problem.
To me the different spaces are more of academic than practical use.
As far as I'm concerned, if it's XML processing in terms of writing SQL queries or PL/SQL code I'll still be directing people to the XML DB forum. That was always the best place before and I think it's still the best place now (as most of the members in that area seem to specialize in that sort of stuff, especially people like Mark Drake).
Having the XML space broken down into three subspaces probably is more than is needed. It could in theory be called the XML - Non XML DB space. Part of that is based on volume as well and people have a tendency to post questions in forums with a higher volume of traffic, simply to get more eyes on their issue. Part of that is in agreement with what Blu said, the forums with higher post volume also tend to have the more experienced people in them. When looking where to put a new post, people will often go to the higher volume spaces, simply because they do not know.
So no, I have no expectation that a newer user will understand which space is 100% correct to put a post, much like I don't expect the experienced users to only be limited to looking at a single space. Could Oracle make it a bit easier, I suspect so, in terms of merging some spaces or clarifying descriptions or providing points to other spaces, much as they did for the PL/SQL XML Programming space. Can they fix it completely? Of course not. I've seen too many questions where people did not even read the error message the database/software provided. And that is the balance between making it easy for the average new user to post correctly while not making it difficult on the experienced user to monitor and respond on his/her set of knowledge.