If you want to add a human readable description of your schemas, use the element "documentation", which is a child of "annotation" element.

Relax NG allows you to place arbitrary elements from other namespaces into your schema.

If you want to combine XML Schema and Schematron (or other) schema language, you can put the Schematron schema into the element "appinfo", which is a child of "annotation" element. Inspired by "Combining the power of W3C XML Schema and Schematron" article.

Relax NG allows you to place arbitrary elements from other namespaces into your schema. I am not sure, if there are any official guidelines, where to properly place the Schematron rules and now it is perhaps implementation dependent. We will place it into "element" element (e.g. SUN MSV accepts this).