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Tracking categories are a feature of MediaWiki that allows pages with certain characteristics to be tracked. It works by adding any such page to a category automatically.
What category to use is determined by the tracking category's system message, that is, a page in MediaWiki namespace. If the message is a single dash (-), then the tracking category is disabled. Often such categories are marked as hidden (__HIDDENCAT__) as they are meant for maintenance and not general consumption. Some sites use Parser functions to modify which category is used based on namespace, but that doesn't work properly until 1.19.
|Default category||Message||What triggers the categories inclusion||Version|
|Pages with too many expensive parser function calls||expensive-parserfunction-category||The page uses too many expensive parser functions (like
||1.13 Before that, Parser Function extension used pfunc_max_ifexist_category|
|Pages containing omitted template arguments||post-expand-template-argument-category||The page is bigger than
|Pages where template include size is exceeded||post-expand-template-inclusion-category||The page size is bigger than
|Noindexed pages||noindex-category||The page is not indexed by robots because it has the magic word
|Indexed pages||index-category||The page has a
|Hidden categories||hidden-category-category||The category contains
|Pages with broken file links||broken-file-category||The page contains a broken file link (a link to embed a file when the file does not exist).
This does not include links to files on the bad image list, files that cannot be displayed inline (due to lack of handler support), links directly to a file page (by preceding with a colon, aka Special:Wantedfiles). The category, if enabled, is linked from Special:Wantedfiles since 1.19.
Extensions can also add new tracking categories (via Parser::addTrackingCategory method).