This is a tracking category.
It is used to build and maintain a list or 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 subcategories 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 tracking category for {{cite web}} CS1 citations where |url= is blank or omitted.
Missing or empty |url=
This error message is reported by {{cite web}}, {{cite podcast}}, and {{cite mailing list}} when the template parameters |url= and |archive-url= are both missing, blank or not properly entered. Note that |website= or |work= is the name of the site, not the URL.
To resolve this error, provide a value for |url= or use a more appropriate template such as {{cite book}}, {{cite journal}} or other Citation Style 1 or Citation Style 2 template.
Pages with this error are automatically placed in Category:CS1 errors: requires URL.[a]
Notes
- ↑ Pages in the Category talk, Draft talk, File talk, Help talk, MediaWiki talk, Module talk, Portal talk, Talk, Template talk, User, User talk, and Wikipedia talk namespaces are not included in the tracking categories. In addition, pages with names matching the patterns '/[Ss]andbox', '/[Tt]estcases', '/[^/]*[Ll]og', and '/[Aa]rchive' are not included in the tracking categories.