The <a4j:region> component specifies a part of the JSF component tree to be processed on the server. The region causes all the a4j and rich Ajax controls to execute: decoding, validating, and updating the model. The region causes these components to execute even if not explicitly declared. As such, processing areas can more easily be marked using a declarative approach.

Regions can be nested, in which case only the parent region of the component initiating the request will be processed.