Caleffi auto fill valve question

russiand

I was looking at a caleffi autofill/backflow preventer combo. The pressure can be set with te screwdriver from the top. However there is an adjusting knob on the bottom as well, and I could not figure out what that does. Any info would be appreciated. It shows a plus and minus if I remember correctly. Does it furter refine the pressure or allow to rapid fill or someting?

Paul48

It's

a shut-off valve.