What's an ID?

Hello,

the id function looks for element nodes with a certain ID value.  But what's 
an ID value in XSLT 2.0?  Is it just an attribute of type xsd:ID or a 
restricted form of it?  If yes, can't you make use of the id function if 
you're using an XSLT processor without Schema support like the open source 
version of Saxon?

Best wishes,
Wolfgang