The Virtual Wall emits a harmless, invisible, infrared beam up to 13 feet long that Roomba regards as a barrier and will not cross. The beam is keyhole shaped and can be as many as 3 feet wide (at the farthest point from the Virtual Wall Unit). You can place it in a doorway to confine Roomba to one room, or it can be used to close off a large open area so Roomba only cleans the area that is soiled.