Category:Content using X template
A category of categories which list pages using a particular template.
This wiki used to have a number of categories like this. By placing a category inclusion into a template, you automatically generate a category populated with pages which use that template. This seems useful, but actually it is often unnecessary, since the "What Links Here" feature can also generate a list of template inclusions. Instead of linking to a category you can link like this:
[[Special:WhatLinksHere/Template:X|your custom text]]
Such categories should probably be deleted, and indeed most of them have now been.