If you using a Symbian 5th edition mobile, you may have experience some times some s60v5 apps and games required Certificate file to sign an unsigned file to start the installation or run the file. All mobiles do not required any certificate or key file. Here is the list of Symbian S60v5 mobile models required Cer. file including all mobile models from other platforms.
Mobile Models: Nokia 5800XM, Nokia N97, Nokia X6, Nokia 5530, Nokia 5230, Nokia 5233, Nokia 5250, Nokia C5, C6, C7, Nokia E7, Nokia N8
This Symbian mobile models photo will also guide you what process is to be done while download / update for certificating a file while online or offline.
Note: One IMEI # use only one certificate file for all installation files.