Beyond W3C XML Schema

The XML developer who needs to validate documents as part of application flow may choose to begin by writing W3C XML Schema for those documents. This is natural enough, but W3C XML Schema is only one part of the validation story. In this article, we will discover a multiple-stage validation process that begins with schema validation, but also uses XPath and XSLT to assert constraints on document content that are too complex or otherwise inappropriate for W3C XML Schema.

http://www.xml.com/pub/a/2002/04/10/beyondwxs.html