My understanding is that the position of the battery switch is immaterial. The Stby Inverter is powered from the 'hot battery bus' and therefore always powered. If you put the Stby power switch to 'BAT' you could then turn off the battery which means the power drain from the rest of the battery circuit is stopped eg if you are miles way from a landing place and can operate without all the Switched battery items you could extend the battery life. You then, of course, could also switch 'OFF' the Stby inverter as well if needed. I think the switched BATT bus has some function involved with 'Auto' Stby power but I'm not sure (hence having it 'ON' normally.)
One of our 'leccie experts will be along shortly I'm sure.