The GND switch is in place to help alleviate ground hum or buzz when the AD-10 is connected to other devices.
When connecting one or more units together, you may sometimes experience a hum or buzz as different devices compete for the electrical grounding.
In these situations, you may need to lift the ground of one, or more devices, to reduce the ground hum.
The GND switch on your AD-10 is specifically designed for the XLR OUT LEFT and RIGHT outputs.
If you hear a ground hum, try setting the switch to LIFT, for ground lift. Otherwise, leave this switch on NOR, normal ground.