Template:Category diffuse/doc

Usage
This is a meta-template; it should only be used within other templates. It is used for namespace detection as a simplification of. If the template is transcluded in the Foo namespace, it will transclude "Example"; if not, it will transclude Incorrect namespace. Note that Incorrect namespace does not work in the template namespace.

The first parameter is necessary. It defines which namespace is the correct namespace for the template.

The second parameter is optional. It allows something (usually a category) to be transcluded in the correct namespace.