Thread:Project:Support desk/Creating maincategories with subcategories in different namespaces and referencing them; or how to use same subcategory names in different maincategories/reply

Categories are a namespace on to themselves. You cannot create other category namespaces, there is only one category namespace. You can do things like and have it be in a category, but that category is still considered part of the category namespace, not part of the Projects namespace. (Note the concept of a namespace in MediaWiki is a bit different from that of namespaces in programming languages, if that's what you're thinking)

Additionally all namespaces should come in pairs of a subject namespace, and a "talk" namespace (with the talk namespace being odd). So if you make a namespace (say number 200) named Projects you have to make another one numbered 201 named Projects talk. (Note, Project (in the singular) is a reserved namespace name in MediaWiki that points to the meta namespace).