Tracking categories are a feature of MediaWiki which allow pages with certain characteristics to be tracked. They work by adding any such page to a category automatically.
Tracking categories can be viewed on Special:TrackingCategories.
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 hyphen-minus (-), then the tracking category is disabled. These categories are often marked as hidden (using the
__HIDDENCAT__ magic word), since they are meant for maintenance and not general consumption.