Canonical representation of QName/NOTATION values

The lexical representation of a QName/NOTATION value has the form 
"prefix:localpart" (or "localpart"), and the value of a QName is a tuple 
{namespace, localpart}. Then what's the canonical representation of a 
QName/NOTATION value? How is the prefix (if any) picked? How is it 
guaranteed that the prefix is always bound to the desired namespace?

Thanks,
Sandy Gao
Software Developer, IBM Canada
(1-905) 413-3255
sandygao@ca.ibm.com