It is also possible to have different towers programed into the zones of an XTS portable so that they can be manually selected. For example, the portable I have at work has Channel 15 as the PVH site and Channel 16 as the Horsetooth site (which leads me to believe that the tower the radio affiliates with can be mode slaved). Being a field/end user, I'm not all up on the technical auspicies of the radio, but I understood that they are programmed with (somebody stop me if I'm wrong) 30 or so different CC's/sites they can affiliate with allowing them to be used freely across a wide area, much like a cellular telephone. If moved anywhere outside of the range of the programmed tower list the radio will display "OUT OF RANGE" (I've seen this message on the display once while in a basement) untill it is able to pick up a signal from one of the towers. As I understood it, coverage outside the programmed tower list is accomplished by simulcasted transmissions on neighboring towers. For example, a message transmitted on the PVH tower would be echoed on the Horsetooth tower. This way if I transmitted a message to the PVH tower and somebody else was using the same talkgroup but affiliated with the Horsetooth tower, they wouldnt miss my traffic. Anyways, dont quote me on any of this, this is just how I understood that it worked.