Tardis:Category tree

Categorisation

Category page descriptions → What categories are not → Category naming conventions → "Real world" versus "from the real world" → Beware recursion → Don't over-categorise → Rule of three → Even good categories can be removed → HotCat → Category tree

Think of all categories as branches on one tree. The goal of our category structure is to make sure that ultimately, all categories go to one place: Floor 500, our top-level category.

Shortcut:

T:TREE

All categories should be nested so that they ultimately lead to one place: Category:Floor 500. Our category tree has only four branches, into which all pages and other categories flow:

With few exceptions, articles that are in one of these four categories should not go into another of them.

Real world and in-universe categories don't mix. Categories like Stories set in 1987 (an out-of-universe category) don't go into things like Category:1980s (an in-universe category). (But see FTRW for more details of how to handle things that actually exist in the real world, but also exist in the DWU.)
If something is "non-DWU material" it cannot logically be in an in-universe category. So Richard E. Grant's Ninth Doctor cannot go into Category:Incarnations of the Doctor, because he's not playing the Doctor as that character is defined at the page the Doctor. He's portraying the same fictional character from a real world perspective, but following Tardis:Valid sources, this says nothing about the fictional universe built outwards from An Unearthly Child which we primarily cover.
Nothing in The Hub should go into an in-universe category. Forum:The Master shouldn't go into Category:The Master, Tardis:Doctors shouldn't go into Category:The Doctor, and so on.