Essentially, the PRL (Preferred Roaming List) contains a list of permitted roaming partners (Perferred Systems) and forbidden systems (Negative Systems) which a CDMA (the type of mobile technology that VZW and Sprint employ) handset uses to determine where it can obtain service from.

By dialing *228 (in the case of VZW), the phone downloads a new (if available) PRL which would add/subtract roaming partners in one area or multiple areas as determined by the provider. It should be pointed out that the PRL is generally national with some rare exceptions.

Hope that helps,
Dan