extension ( XML Representation Element Term )
extension [ XML Schema Namespace ]
Subject Indicators
Used to extend the content model of an existing complex type. When deriving a new complex type by extension, the content model of the derived type is considered to be a sequence consisting of the content model of the base type followed by the content model of the extension. An extension type will be valid in any content model where its base type is valid, however the xsi:type attribute must be used in the instance to indicate the extension type being used.