Please go read this very long article about classification: Ontology is Overrated: Categories, Links, and Tags. I know it looks like it’s about computers, but it is also about libraries and tags, and sense-making and why you can’t gracefully take library classification schemes and slap them on to web pages. Go. Go now and read and learn.
It’s tempting to think that the classification schemes that libraries have optimized for in the past can be extended in an uncomplicated way into the digital world. This badly underestimates, in my view, the degree to which what libraries have historically been managing an entirely different problem.
It comes down ultimately to a question of philosophy. Does the world make sense or do we make sense of the world? If you believe the world makes sense, then anyone who tries to make sense of the world differently than you is presenting you with a situation that needs to be reconciled formally, because if you get it wrong, you’re getting it wrong about the real world…. If, on the other hand, you believe that we make sense of the world, if we are, from a bunch of different points of view, applying some kind of sense to the world, then you don’t privilege one top level of sense-making over the other.