Manual:$wgContentNamespaces

Details
exists to allow custom namespaces to be acknowledged as containing useful content for the purpose of the following features:

Extension:Popups, Special:Random, Special:Statistics, Special:AncientPages, Special:DeadendPages, Special:FewestRevisions, Special:LonelyPages, Special:MostCategories, Special:MostInterwikis, Special:RandomPage, Special:ShortPages, Special:LongPages, Special:UncategorizedPages, Special:WithoutInterwiki,

Useful if additional namespaces also contain "content" which should be considered when choosing a random page or generating a count of the number of articles in the wiki. It also serves as the default for, which means that __NOINDEX__ and __NOFOLLOW__ magic words won't work in these namespaces.

The main namespace is considered a content namespace, as are other namespaces in this array, referenced by index.

Example:

or

This does not control which namespaces the full-text search engine defaults to searching; that must be configured separately using.