As you can probably imagine, the drive control on Sono allows you to add overdrive to your signal, giving you saturation and breakup, great for lead sounds!
This overdrive is added in the Two Notes CAB processing of Sono and emulates the saturation of power amp valves. The Drive control doesn't adjust the saturation of the valve stage or add any overdrive in the analogue domain.
This is done so that Sono can always allow you to capture a clean DI at the same time as capturing your processed guitar signal. More information about this can be found here.
When changing between presets, the drive amount will also be recalled and therefore the position of the knob may not be accurate to the amount of drive applied. In this instance, the drive control would need to be moved by approximately a quarter of a turn before the Sono will override the presets drive level.