If you specify this event and it is enabled, Hollywood will always jump to this label when it gets an ONBUTTONRIGHTCLICK event. If you want to know, which button caused the label jump, just call GetEventCode(). It will return which button caused the event.
This event is very useful if you have e.g. 100 or more buttons and every button just does a Gosub() to a sub-routine which receives an id which button was pressed. Instead of defining 100 labels now, you can just use ONBUTTONRIGHTCLICKALL together with GetEventCode().