See: http://www.w3.org/XML/Group/2007/09/scds

Provide explicitly for extension to SCDs in the form of:
* new axes (in XPath, the axis names are part of the grammar; but there is a distinct grammatical construct from which all axis names are derived; introducing a new axis name thus involves minimal derangement of the rest of the grammar)
* extension functions (e.g. to allow implementations to define accessors for properties not exposed by accessors in the SCDs spec); in XPath, the grammar suffices to recognize function calls; the list of known functions is part of the context, and can be extended by implementations, or by users under the right circumstances. The ability to define extension functions has worked well for XPath; we commend this model to you.