Schematron 2025 is a validation language that checks XML documents against business rules. Based on the ISO Schematron specification from 2025, it extends the validation provided by XML schema languages such as Document Type Definitions (DTD), W3C XML Schema, and RELAX NG to check XML documents for compliance with rules that can be difficult, if not impossible, to check with the other validation languages.
Schematron 2025: A Language for Validating XML explains the language in detail and clearly outlines the differences between Schematron 2025 and the previous edition, covered in Erik Siegel's previous book: Schematron: A Language for Validating XML.