Schematron is a simple and powerful Structural Schema Language for making assertions about patterns found in XML documents. It relies almost entirely on XPath query patterns for defining rules and checks. Schematron validation rules allow you to specify a meaningful error message (as opposed to a cryptic error code) which will be provided to the user if an error is encountered during validation stage.
Oxygen XML Editor plugin uses for validation the Skeleton XSLT processor and conforms with ISO Schematron or Schematron 1.5. It allows you to validate XML documents against Schematron schema or against combined RELAX NG / W3C XML Schema and Schematron.
Oxygen XML Editor plugin assists you in editing Schematron documents by providing schema-based content completion and syntax coloring. A basic Schematron template is available in the New Document wizard. The Schematron editor renders with dedicated coloring schemes the XPath expressions. You can customize the coloring schemes in the preferences page.
Any time you can validate the content using the Validate action. Another way to validate schemas is to check them against their own Schematron schema rules using External validation action.