The different settings for the autofocus will constrain the range over which the lens tries to focus on. For example, 1.2m -inf will try to autofocus starting from 1.2m up to inf if it can't get a lock. This is useful in situations where you are far away from the subject (sports) and you know you aren't going to be closer than 3m. Thus if you set the AF range to 3m - inf, you can get a faster focusing speed as the lens does not have to "turn" too far to get the right focus, as opposed to 1.2m - inf.