The PRL file is basically a list of towers that are ok for your phone to use. i'm not sure how often cricket updates this list, but sprint and verizon usually update this 3-4 times a year. updates will contain any new towers that have been added and remove and towers that are no longer working. you only need to update this if you are having a problem with reception in an area you think you should have reception. dialing *228 will download the latest PRL from cricket, among other things.