Thread:Jay0110/@comment-34326521-20140107221348

Categories within a category must apply to every page the second category belongs to. In other words, if you're going to add categories to the "Aliens" category, that category you wish to add must also belong to every page the "Aliens" category belongs to.

In addition, don't add categories that don't exist. If you want to create a category, use the creation portal before adding that category to pages, but note that categories that are deemed bad, for whatever reason, will be deleted. 