This activator could be defined only in bound with a region created using plugin WorldGuard. To create activator of this type you need to use command:

/react add REGION_LEAVE <ActivatorId> [<World>.]<RegionName>

REGION_LEAVE activator is activating when player leaves a region. It activates only one time. When player enter region and leave it again activator will be initiated again too.

ReActions uses a format "[<World>.]<RegionName>" to determine region. You can skip the world name if you region located at your server main world.

Video tutorial