The "VS" screen tells you the voltage of a second bank. Seeing as I used one battery for this illustration the meter is showing the same voltage as the the "V" screen.
I should mention that there have been a couple of people on the boating forums complaining that the V & VS screens, even when fed from the same source voltage are not in sync or well calibrated. This sample happens to be properly calibrated. An experienced sailor and electrical engineer on SailboatOwners.com has had two units with voltage readings from .01 - .03 volts off when sensed from the identical source.
Is a variance of .01 to .03 volts a big deal? No, not at all, but I just wanted to make you aware so that you don't panic if your V & VS screens do not completely agree. Victron might need to do some better QC with the V & VS screen readings!