Manual:$wgExtraNamespaces/en

Details
The  configuration parameter lets you configure additional, custom. It can also be used to rename the default namespaces. See the manual on for further details.

Developer information:

In case an extension should work with the newly created custom namespace, make sure that you define and name them prior to invoking the respective extension.

Extensions should not use this configuration parameter. Instead they should use the hook or load the namespace through.