This section describes each of these techniques and how they can be used. In these descriptions, we use the term referenced XML Schema to indicate the XML Schema that is being included, imported, or redefined; and base XML Schema to indicate the XML Schema in which the referenced schema is being included, imported, or redefined.
Referencing an XML Schema by redefining it is similar to including it, with one important difference: when you redefine an XML Schema in the base XML Schema, you can redefine the definitions of the referenced XML Schema's complexTypes, simpleTypes, groups, and attributeGroups. For example, suppose you release version 1 of an XML Schema. When you need to release version 2 of the XML Schema, you can reference version 1 by redefining it in version 2, which allows you to change the definition of a given node to include a new attribute.