About the preferred wireless network list
Your wireless service provider might have agreements with other wireless service providers that permit you to use their services when you are traveling. You can use the preferred wireless network list to control the order in which your BlackBerry® device uses these other wireless networks.
Your device tries to connect to a wireless network based on the order in which the wireless networks appear in your preferred wireless network list. When you travel, your device scans for the wireless networks on this list first. If no wireless network on the list is available, your device is designed to connect to another available wireless network automatically.