Extensibility

Unlike a Document Type Definition (DTD), XML Schemas are extensible. That is, they are built on an open content model. Schema authors are free to add their own elements and attributes to XML Schema documents. For example, you could utilize this extensibility mechanism to add additional constraints to a declaration for a "pages" element. The following sample code declares the "pages" element and assigns it an "int" datatype. Extended tags from the "myExt" namespace are used to augment this information with an added limitation that books must have a minimum of 50 pages and a maximum of 100 pages.

<ElementType name="pages" xmlns:myExt="urn:myschema-extensions">
  <datatype dt:type="int" />
  <myExt:min>50</myExt:min>
  <myExt:max>100</myExt:max>
</ElementType>

While validation will only check that the value of a particular "pages" element is an integer, your application can use the information provided by the added elements in the "myExt" namespace to perform additional validation.

It's worth noting that the elements that Microsoft added to the schema were namespace qualified. Use of a namespace is a requirement for adding elements and attributes to a schema. For more information on namespaces, see Introduction to Namespaces.