The vast majority of transmission fluid leaks off the side of the trans are from the flange cap (leaks into the CV) or drive axle seal (leaks down side of trans). On rare occasion the leak down the side of trans comes from this 0-ring which is behind the threaded side cover. This cover requires special tool to spin loose and when tightening must be set correctly (although you could potentially mark it and spin it back to where it was). This is NOT where leaks usually come from on side of trans, but when they do we offer the seal separately. Price is for one seal.