This sets the DC bus voltage threshold that, once the battery voltage sags to this level, disconnects load bank 2 from the bus to protect the batteries from over-discharge, per the description. Because the plant lets each of its three load banks disconnect at a different voltage, bank 2 is typically configured to disconnect after bank 1 but before bank 3, sitting at a threshold usually set lower than disconnect1Voltage but higher than disconnect3Voltage. A setting of 420 (42.0 volts) means load bank 2 drops off the bus as soon as the battery sags to 42.0V during an extended AC outage.