Are you talking about the secondary idle adjustment screw? If so it allows you to adjust the idle on a car with low vacuum/big cam, without uncovering too much transfer slot in the primaries.
INSTEAD OF RAISEING THE IDLE WITH THE PRIMARY ADJUSTMENT SCREW I CAN RAISE IT MORE IF NEEDED WITH THE SECONDARY SCREW.
900 RPM WITH PRIMARY & 200 MORE WITH SECONDARY.
I like to take the primary to a minimum position and then adjust the secondary to get the RPM I want. I think the carb is more responsive off idle that way. Just my opinion.