This is a maintenance category, used for maintenance of the Wikipedia project. It is not part of the encyclopedia and contains non-article pages, or groups articles by status rather than subject. Do not include this category in content categories.

This is a hidden category. It is not shown on its member pages, unless the corresponding user preference (Appearance → Show hidden categories) is set.

This is a tracking category.

It is used to build and maintain lists of pages—primarily for the sake of the lists themselves and their use in article and category maintenance. It is not part of the encyclopedia's categorization scheme.

More information:

This category is hidden on its member pages—unless the corresponding user preference (Appearance → Show hidden categories) is set.
These categories are used to track, build and organize lists of pages needing "attention en masse" (for example, pages using deprecated syntax), or that may need to be edited at someone's earliest convenience.
These categories also serve to aggregate members of several lists or sub-categories into a larger, more efficient list (discriminated by classifications).
Typically, tracking categories are automatically populated by templates invoked in pages or by the MediaWiki software (see Special:TrackingCategories for an overview of the latter).

Administrators: Please do not delete this category as empty!
This category may be empty occasionally or even most of the time.

This is a category to help keep count of the total number of articles with the ((Data missing)) template. They should all be in one of the dated categories. See also Category:Data missing.