XSL extensions live in a specific namespace. Users can define their own prefix to associate with a given namespace, but in general sticking with a consistent naming convention makes life easier.

For example, the XSL namespace is:

To define a prefix for it, one adds an XSL namespace declaration to your XSL stylesheet:

Now the desired namespace can be referenced just using a prefix rather than the actual namespace. Therefore, the following two lines are equivalent:

Microsoft's XSL transform implementation does not support extension elements at this time. Therefore, methods must be called from within "select" contexts.