Determines if your phone will use a 3G or 2G mobile data connection.
If you set it to On Toggle on icon, your phone will use a 3G connection when it's available. When this is set to Off Toggle off icon, your phone will use a 2G connection which is a slower mobile data connection, but can also help you to save a little battery power.
This setting only appears on dual-SIM Windows Phones, which run on Windows Phone 8.1.