Isn't a bit late to say that "Important: The level map is not (yet) accepted as a standard means of mapping. This page is to support the discussion whether or not to use it. So please do not use this instructions for your every-day mapping."
I and others have been using this as a mainstream tagging for some years now especialy in complex buildings and features builts over and under each other!
gives places where it is in major use already!
Indoor mapping page has a version 2.0 too!