Extension default namespaces
www.mediawiki.org/wiki/Extension_default_namespaces
What Jamie wrote
Learning a little too late that 100-199 is the user namespace area.
The namespaces in 100-199 are reserved for site-specific namespaces, and should not be used by extensions. A few extensions have apparently used this area, so be careful using them. If you're writing a new extension you should never hardcode any namespaces inside the 100-199 range.