Content namespaces
A content namespace is a namespace that contains content pages. By default, the only content namespace is the main namespace, or "mainspace." You can see what pages are in the main namespace of a wiki if you go to Special:AllPages. By default that page lists mainspace pages, and you can choose a different namespace from the dropdown menu.
Use cases for custom namespaces
If your wiki covers multiple games, and these games overlap in their content a lot, you might want to use multiple namespaces. For example, if your wiki is about League of Legends and Teamfight Tactics, which share playable characters ("champions"), you might want to create League of Legends: and Teamfight Tactics: as separate namespaces. This provides a built-in way to avoid disambiguation issues.
Alternatively, if your game has a section of community guides or fanfiction, maybe you want to create Guide: or Fanfiction: to denote that this content is not canonical.
Reasons not to use custom namespaces
Generally, however, you want to avoid custom namespaces. There are a few reasons:
- Complication for new editors - because namespaces are separated in places like Special:AllPages, Special:MovePage, and others, you might confuse editors who can't find their content.
- Complication for filtering in code - see this article about restricting certain actions to certain namespaces. With multiple namespaces, this becomes more complicated.
- Complication with search - while it's possible to edit the set of default namespaces that show up when you go to the page of search results, only mainspace articles will show from the search dropdown. This is not configurable.