externally-determined ID implementation experience?

I just discovered

"[Definition: An externally-determined ID is a string, representing an
element identifier, whose value is determined by the application through
mechanisms outside the scope of this specification.]"
-- http://www.w3.org/TR/2003/REC-xptr-framework-20030325/#term-xdi

I'd like to know about some examples that motivated
that feature.

Could you point me to an item in a test suite or an
implementation report or whatever where implementation
experience with this feature was demonstrated?

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
see you at the WWW2004 in NY 15-21 May?