What is this?

My new toilet has this and I am not sure what the white screw does. The toilet will run a little from time to time but just goes into the white hole and strait down the drain.(I can hear it going down sewage pipe in basement) So I am guessing I am wasting a lot of water. But when I flush it actually goes into the white hole to fill the bowl.

I'm almost positive it's the adjustment for the water level. Not sure which direction is up/down.
What brand of toilet?
That black tube from the ballcock should be in the overflow tube to refill the bowl.

I believe it is a whacky Kohler flush valve.
Similar or maybe same as here.

https://www.amazon.com/KOHLER-1208883-Flush-Valve-Unfinished/dp/B00ODOABXK

White screw is either for adjusting how much water flushes before the flush valve shuts, or else how much water fills the bowl. But most likely how much volume of flush.

Yes, that black tube brings over water to refill the bowl, it should pbly go down that skinny round white opening above the white screw.
But this is a whacky dual flush type valve.
So maybe the black refill hose just dumps water into the tank, because they used a generic fill valve, and this system might handle bowl refill water level from within this goofy flush valve.

So that white screw might also control flush volume for pee flush or for poo flush. Call Kohler, unless someone here has the inside scoop.