From Joomla! Documentation
These Wiki pages are used for the navigational boxes of articles, which aid in navigating the Wiki and its namespaces, and cross-linking pages in series.
This category could contain pages in the template namespace, Main namespace subpages, chunk, etc. It must not be used to categorize articles or pages unless the page is specifically used in transclusion to provide a navigational box or links to articles in a series or tutorials.