Earlier quoted context omitted.
Knowledge doesn’t neatly align to a nested hierarchy. Especially written knowledge. Language is an imperfect means to convey knowledge, and people store that knowledge in subjective and highly personal ways. You may mentally recall balloons within “entertainment” or “party”, whereas I might store that knowledge under “horror”. Add onto that the massive focus on using graph theory to scale social networking technologi…
> Knowledge doesn’t neatly align to a nested hierarchy. Especially written knowledge. The category tree being displayed comes directly fron wikipedia. E.g. Wikipedia has pages like https://en.wikipedia.org/wiki/Category:Art
Since this is so terribly impossible most systems almost immediately make it possible for things to show up in more than one place, which means it's actually hierarchial tagging, whether or not the organizer(s) realize it.
You could also make a distinction based on how many tags things end up with; if it's almost always one, you could call it a nested hierachy with some exceptions, but if it's almost always more than one, and often much, much more than one, it's a tagging system. Even by that criterion that creates a spectrum rather than a binary distinction, Wikipedia is very much organized by tags and not hierarchies. I don't know what the average is but every Wikipedia page I've ever looked at the tags for has quite a few.