This section is just a primer. The reference documentation about this topic is really Section 25, “relaxng” in XMLmind XML Editor - Configuration and Deployment.

A document type declaration (<!DOCTYPE>) can be used to associate a DTD to a document. Attributes xsi:schemaLocation/xsi:noNamespaceSchemaLocation can be used to associate W3C XML Schemas to a document. But there is no standard way to associate a RELAX NG schema to a document. Therefore this association must be made using an external specification such as the Namespace Routing Language (NRL).

In the case of XMLmind XML Editor, this external specification is simply a configuration element called relaxng.

XHTML example:

<configuration name="XHTML Strict [RELAX NG]"
  xmlns:html="http://www.w3.org/1999/xhtml"
  xmlns="http://www.xmlmind.com/xmleditor/schema/configuration">
  <include location="xxe-config:schema/ns_xhtml.incl" />

  <detect>
    <rootElementNamespace>http://www.w3.org/1999/xhtml</rootElementNamespace>
  </detect>

  <relaxng location="xxe-config:common/rng/xhtml1/xhtml-strict.rng" />

  <preserveSpace elements="html:pre html:style html:script" />

  <css name="XHTML" location="xhtml_rng.css" />
  <template name="Page" location="page.html" />
</configuration>

The <?xml-model> processing instruction allows to associate schema documents written in any schema definition language with a given XML document. As such, it may be used to associate a RELAX NG schema, written using either the XML or the compact syntax, with a document. Example (excerpts from name.xml):

<?xml version="1.0"?>
<?xml-model href="name.rnc" type="application/relax-ng-compact-syntax"?>
<names>
  <name><fullName>John Smith</fullName></name>
  ...