In XML validation, there are two primary approaches to ensure the accuracy and integrity of content:

Structural validation (e.g., RELAX NG, XSD) handles the overall structure of an XML document, ensuring that elements appear in the correct order, adhere to the proper data types, and follow hierarchical relationships between elements. For instance, when a bibliography element is present, the following-sibling must be as per the following: expected the element end-tag or element "bibliography", "glossary", "index" or "toc".

On the other hand, Schematron enforces a more granular, rule-based validation that can be precisely tailored to business requirements. For instance, for accessibility purposes, when figure elements are present, alt elements must be included as a child of the mediaobject element, or the file will raise error messages.

<figure @xml:id="b-figure1"> 
      <info> 
        <title>Image</title> 
      </info> 
      <mediaobject> 
        <imageobject> 
          <imagedata fileref="images/f001.jpg" format="image/jpeg"/> 
        </imageobject> 
      </mediaobject> 
    </figure>

Both structural validation and Schematron serve different but complementary roles; combining them ensures that the XML content adheres to a required structural format and meets the customised requirements. This multi-validation approach significantly improves the reliability and validity of content throughout its lifecycle, ensuring that the high standards required in publishing are met. Additionally, we use the parsing results from industry standard validators for each flavour of schema, where any such errors should be reported by them as per usual.