Overview of Schematron
Since May 2006 the ISO Standard Schematron is part of the Document Schema Definition Languages (DSDL). Schematron is used for the validation of the structure and the content of XML documents. The schema language allows to check business rules which cannot be done with other languages.
Schematron is a supplement to other languages as DTD, XML Schema or RELAX NG and may be used in addition to parsers.