It should be set to whatever values you want for your particular battery.
It reads 0, because 9V is empty, and you’re reading at 8v. What kind of battery pack do you have on there, and does that voltage match your battery as measured with a multimeter? If not, then you need to calibrate your voltage reading (on the Initial Setup->Optional Setup->Battery Monitor screen)
If you have a 3S, you may want it to read 11.2,11.5,12.6, depending on how your particular battery pack voltage comes out when discharged, and where you want the warning value, 20%, 35%, and 100% here for instance.