Tardis:Category 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 Rowan Atkinson'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 playing a parodic version of the Doctor who isn't a part of the DWU.
 * 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.