Those screws marked H and L on a deraileur are limit screws. They physically limit the motion of the deraileur, functioning as adjustable hard stops of the outer range of the deraileur’s motion. One prevents it from over shifting the chain between the cassette and into the spokes, the other from pulling the chain past the smallest cog.

The L for “low gear” screw limits the motion of the deraileur into the largest cog, the H for “high gear” screw limits the motion past the smallest cog.