There is a grey button-like thing on the right side of the New Nintendo 3ds XL. It is above the main ABXY buttons:
Specifically, it's on the bottom half of the system, next to the screen, straight above the Y and diagonal to the X button. I used to have a normal 3DS, but I don't think this ever existed on it.
It looks like a button, but I'm not sure. What is it and what does it do?