Extension, Restriction, Prevention

Inheritance issues While the modularity of types is attractive, inheritance features add much-needed flexibility.
Adding and Subtracting The derivedBy attribute lets you specify extension - in which case you can add to the definition - or restriction - which lets you remove parts. (Restriction is still in limbo, moving slowly toward stability.)
Equivalence XML Schemas lets you declare different types equivalent with the equivClass attribute. This makes them more interchangeable, somewhat like objects of a common parent type, though no common parent is required.
No children, please! The final attribute for complex types allows you to prevent all derived types (#all), extensions (extension), or restrictions (restriction).