the prl tells your phone how high up on the priority list your phone is for pingingn off the towers. your phone and the tower work harder to make you number 1. for those not in the know that don't select prl update every now and again, and those that have had the same flip (clam shell) device for last ten years running prl 51101 (not really, but)........
kevmo1965 posted a link that pretty much says that.