This part of ISO/IEC 19757 specifies Schematron, a schema language for XML.

Considered as a document type, a Schematron schema contains natural-language assertions concerning a set of documents, marked up with various elements and attributes for testing these natural-language assertions, and for simplifying and grouping assertions.

Considered theoretically, a Schematron schema reduces to a non-chaining rule system whose terms are boolean functions invoking an external query language on the instance and other visible XML documents, with syntactic features to reduce specification size and to allow efficient implementation.

Considered analytically, Schematron has two characteristic high-level abstractions: the pattern and the phase. These allow the representation of non-regular, non-sequential constraints that Part 2 cannot specify, and various dynamic or contingent constraints.

This part of ISO/IEC 19757 establishes requirements for Schematron schemas and specifies when an XML document matches the patterns specified by a Schematron schema.