Could also be scale of classification. We call humans, dogs, rats, coral, and sponges, all animals. In that case we group them according to kingdom. Maybe monster is a blanket term for a bunch of things, possibly higher order than kingdom, though most likely lower than domain, thus making it BM's fictional second highest order of classification.
I say that it must be lower than domain because I am pretty sure humans couldn't tell the difference between bacteria and archaea at this time. I say that it is most likely higher than kingdom because I believe that in talking about "monsters", there are not only animal monsters like those humanoid mammalians on BT, but also plant monsters like those fantasy carnivorous plants. And if not yet, then there should be. That would explain a lot of things that happen in the forests of midsouthwest Dwilight.