SchematronLanguage is a rule based validation mechanism for XmlDocuments?, and soon to be (in 2005) adopted by IsoOrganization as an international standard. See http://www.xmlmania.com/news_article_1669-Final-Committee-Draft-of-ISO-Schematron-Released-for-Public-Review.php
It is said SchematronLanguage can be used in conjunction with other SchemaLanguagesForXml.
Resources
overview http://xml.ascc.net/resource/schematron/schematron.html#overview
(2002) Use with XmlSchema or RelaxNg http://www.topologi.com/public/Schtrn_XSD/Paper.html