It is an idle air adjustment screw. It is usually set by the manufacturer and doe not need to be adjusted. It adjusts how much air bypasses the idle air control valve to help stabilize idle rpm.
That's what I thought. My issue with my 5.0 Thunderbird is the stock idle (~675 RPM) is so low, that when coasting in cruise control or manual at speeds above 50 mph my tranny will shudder. The lightest application of throttle and it'll "pop" back to normal. So I found the solution by adjusting the set screw on the throttle stop to raise that base RPM just enough to not make the tranny shudder. It's a little bit trial and error.. Seems my torque convertor's stall speed may be triggered at that low of RPM...
So I've got it stopped by moving the set screw enough that it's gone, the base RPM now is more like 750-800 RPM, but those adjustments kind of mess with the TPS a little, so I'm hoping it'll settle down and learn the new base RPM...
However, if I could adjust the idle with that screw and not the set screw on the throttle stop, that could make for a more stable idle...
But I'm not messing with it, if there's no instructions out there...