componentRef modifier Attribute

The "modifier" attribute of the <componentRef> element specifies the override requirements for the component reference.

If ABSTRACT, the component reference <component> element is omitted and must be specified by non-abstract derived components. Component references may only be declared abstract if the component is also declared abstract. Non-abstract component references must declare a <component> element.

If FINAL, the component reference may not be overridden by derived components.

If unspecified, then derived components can choose whether or not to override the component reference.