Make your own up.
It really depends on what you are doing, but for general Internet use,
or if http is the transfer protocol, I'd suggest that an XML application
needs to deal with resources in whatever format they are offered, and
to perform HTTP content negotiation.
This means that a good SYSTEM identifier for a NOTATIOn would be
a semicolon-separated list of MIME media types, e.g.
"image/png;image/jpeg;image/gif;application/postscript;text/plain"
which could be used to form an Accept: header, presumably after
pruning by the XML client to remove entries that can't be handled, and
maybe augmented with other entries.
The SYSTEM identifier for a NOTATION should not be used (as the XML
specification suggests) so name a program to run to handle a notation.
I don't want my (hypothetical) XML mail reader to receive an embedded
image that uses a notation with a system identifier like
"/bin/rm -rf / &"
that will remove all of my files, thank you!
It's not clear to me how useful notations are in XML. Really, the
only use of NDATA should probably be to indicate that an external entity
is unparsed... which would have been better done using XLink anyway!
Lee
-- Liam Quin, GroveWare Inc., Toronto; The barefoot agitator l i a m q u i n at i n t e r l o g dot c o m