a puzzling schema promise

["Simon St.Laurent" <simonstl@simonstl.com>]

Press releases are often a better place to find an explanation of what 
specifications are meant to do than the specifications themselves.  The 
W3C in particular has had strong press operation for a long time, and 
generally conveyed its plans for specifications pretty well.

Based on that theory, I've been exploring the archive of schema-related 
press releases, and found this odd sentence.  The opening is just the 
usual prior agreement promise, but I'm not sure what to make of the 
conclusion:

--------------------------------------------------
When XML is used to exchange technical information in a multi-vendor 
environment, schemas will allow software to distinguish data governed by 
industry-standard and vendor-specific schemas, and help applications 
know when it is safe to ignore information they do not understand, and 
when they must not do so.
--------------------------------------------------

This sounds more... intricate than validation, but I'm guessing that's 
all it really is.  Is there something more to "when it is safe to ignore 
information they do not understand, and when they must not do so"?

That last bit seems to expand to "must not [ignore information they do 
not understand]".  That fits quite nicely in the approach I'm describing 
- but not so much with any XML Schema application I've encountered.

Any thoughts?

Thanks,
-- 
Simon St.Laurent
http://simonstl.com/