a4j:region component designed for marking zones which should be processed during Ajax requests without using execute definitions at Ajax behaviors/components. This allow to decouple the processing definitions from components id's and use declarative wrapping by this tag instead.

In RichFaces 3.3 all the view was processed if no regions specified as ViewRoot itself was region by default. And in RichFaces 4.x - all the controls has execute="@this" by default according to JSF 2 specifications. But if the controls wrapped with a4j:region component and has no execute definitions - them uses execute="@region"

Look to the simple example below. There are two similar user info panels. But the button inside the first panel will not works as not uses execute declarations and by default execut equals to @this. Separate definition needed to start working properly. And the second panel inputs just wrapped to region with the commandButton and works without any additional execute definitions.