XML in a Nutshell, 3rd Edition

Simple Content

Earlier, the xs:simpleContent element was used to declare an element that could only contain simple content:

<xs:element name="fullName">
  <xs:complexType>
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute name="language" type="xs:language"/>
      </xs:extension>
    </xs:simpleContent>
  </xs:complexType>
 </xs:element>

The base type for the extension in this case was the built-in xs:string data type. But simple types are not limited to the predefined types. The xs:simpleType element can define new simple data types, which can be referenced by element and attribute declarations within the schema.