4.5 Component Extension and Refinement
A component classifier or port group type declaration can extend another component classifier or port group type.  This extension can refine previously declared model elements or add new model elements.  This extends relationship is represented by an extends reference association of each category-specific component type or implementation or a port group type to itself.  Figure 19 shows this reference relation for the process type, process implementation, system type, and system implementation.
The extends reference association represents an extension inheritance relationship.  The model elements being inherited are not explicitly replicated in the persistent XML representation of the extension.  Instead extension inheritance is used to keep the size of AADL object models and XML documents smaller.
Refinement of previously declared model elements in component types and implementations is represented by a refines reference association of each concrete subclass that is acceptable content to component types, component implementations, a port group types.  Figure 19 shows the refines reference association for the ProcessSubcomponent and SystemSubcomponent class.  The object representing the refined model element only contains the refinements in its persistent XML representation.  Methods on the AADL object model provide access to the refined as well as the inherited attributes and associations of the refined object.
The extends reference association and the refines reference association are defined for each category to ensure that the references are to objects of the same class.