O.k. I'm with you, you're talking about the DTD's. I just wouldn't call that proprietary because..well..that's the point of DTD's. They're supposed to be customized. My impression from your post was that the XML specification (or components related to, such as DTD) had been changed to suite that company. Creating a DTD to specify what data is valid and what isn't is, of course, why XML is as good as it is.

NOTE:
For those who have no idea about XML, a DTD is a "Document Type Definition" which specifies how your data will be structured, what's valid, what's required, etc. XML::Parser and other _validating_ parsers will check an XML document against the specified DTD to ensure that it meets that specification, not all parsers do that (i.e. non-validating parsers). This is very well suited to many things, and is largely becoming an excellent way for different businesses to exchange information.

NOTE2:
We're all so darned geeky here that I'm sure nobody got anything out of that ;). Just lookin' out for the potential new guy.