GND is the device ground connection. The device can operate with a ground shift as long as the ground shift does not reduce the V_SUP below the minimum operating voltage. If there is a loss of ground at the ECU level, the device has ultra low leakage from the LIN pin, which does not load the bus down. This is optimal for LIN systems in which some of the nodes are unpowered (ignition supplied) while the rest of the network remains powered (battery supplied).