How does Nikon D600 choose focus?

First, press the round button between af and MF keys with your left hand, and then turn the front and rear wheels with your right hand. After switching to AF-S, you can choose the focus according to your own understanding. Before choosing the focus, you must unlock it, that is, there is a circle up and down, left and right. If it is aligned with l, it is locked. Go to a position other than l first, and you can choose.