This is the washer in question. It is used to refine the angle of the derailleur in the plane of the bike. Its goal is to solve minor not major problems. In my memory, that Sram washer rotates the derailleur 5 deg in the clockwise or counterclockwise directions. Similar washers on Aliexpress correct the angle by 4 deg, again in my memory. That angle may be reduced by some filing.