Content Model

You can specify the content model for an ElementType by using <element> to reference other ElementTypes. In the example below, "title" and "author" elements are referenced within the definition of the "book" element. This allows you to write the following XML:

<book>
  <title>Presenting XML</title>
  <author>Richard Light</author>
  <pages>334</pages>
</book>

This is a very simple case. There are a number of mechanisms in XML Schema that allow you to build more complex content types.

The most important innovation for content models in XML Schema is that content models are "open" by default. An open content model enables additional tags to be present within an element without having to declare each and every element in the XML Schema. This provides an extensibility mechanism not present when using a Document Type Definitions (DTD). In instances where an open content model is not desired, the model attribute can be used on the ElementType. For example:

<Schema xmlns="schemas-microsoft-com:xml-data">
  <ElementType name="book" model="closed"/>
    <element type="title" />
    <element type="author" />
    <element type="pages" />
    
    <AttributeType name="copyright" />
    <attribute type="copyright" />
  </ElementType>
</Schema>

This indicates that a "book" element can only contain the content specified—"title," "author," and "pages" elements.