@clintflint - When you consider that library science is actually a degree at university where you are basically taught Dewey decimal system classification (among other things, of course) it's not surprising that you had difficulty. Any system that can categorize every topic in human thought and experience is going to be very dense and difficult to interpret, although the Dewey system is as good as anything I can imagine.
I wonder how often (if ever) they get a new topic that needs to be decided on. Something that could go in more than one area, for example.
I wonder if there's a place where they decide these things, or if it's up to each individual library.