NB. This is a concept, an idea, a plan, a design. Not implemented yet.
Currently, categories are all on the same level. You can't put a category into a category. But what if you could? This is basically the idea of subcategories.
The idea is as follows. On category pages, there is the same category sidebar like on hypha pages. Thus, you can nest categories.
How to enforce the tree structure? I find it hard to do in UI.
Should the tree structure be enforced? Media Wiki does not do that.
Categories may belong to other categories in a hierarchy. Since category pages are much like any other page, a Category tag may be added to the bottom of a category page.
It is a good idea to organize all categories into a hierarchy with a single top level category. The category structure can take the form of a tree with separate branches, but more often will have a graph structure. Generally, there should be a contiguous chain of parent-child links between each category and the top level category.
It allows graphs, and delegates the arborification to the users. Avoid responsibility.
Wouldn't it repeat the problem of nested hyphae? What if we limit the maximum level of nesting to, maybe, three?