XML schema specification version 1.1  specifies syntax and semantics of "Overriding component definitions<override>" or xs:override , under "Schemas and Namespaces: Access and Composition" . The new component definition is a powerful addition to the XML schema composition framework ,which tries to mitigate some of the constraints present in similar constructs such as xs:redefine or <redefine> schema component.
The <redefine> construct defined in "Including modified component definitions (<redefine>) " on XMLSchema specification is useful in schema evolution and versioning. It can be used only when there exists some restriction or extension relation between the old component and the new redefined component. But there are occasions when the schema author simply wants to replace old components with new ones without any constraint. Also, existing XSD processors have implemented conflicting and non-interoperable interpretations of <redefine>. The implementation of new construct <override> on Xerces XML Schema processor ,would avoid these bottlenecks and allow unconstrained replacement as and when needed by the schema author.
Possible Mentors: Khaled Noaman