If you are talking about the little dial at the back then it is a limiter. If you screw it in, is blocks the needle from moving back. The further you screw it out, the more the needle can move. You limit the movement so that when you have a desired thickness to repeat, you set it and no matter what, you can not go wider. Except if you move slower or you hold the gun further away from the surface. I still need to find a place where I will use mine.