XSD-based standards using abstract elements?

XBRL and XrML are two standards I know of that, instead of defining a group
of elements for conforming documents to use, define abstract elements so
that schemas conforming to the standard have a structure on which to base
elements that they must define themselves. (The strange noise you hear is
the ghost of architectural forms dragging chains around the attic.) Does
anyone know of other XSD-based standards that do this? 

thanks,

Bob DuCharme          www.snee.com/bob           <bob@  
snee.com>  "The elements be kind to thee, and make thy
spirits all of comfort!" Anthony and Cleopatra, III ii