Focusing range switch

[Blue Deuce]

What is the real world purpose of using the two different settings on my 100-400mm ?

[tim]

The higher numbered range saves time if the camera hunts for focus.

[ttmatsu]

Stand closer than the minimum focusing distance your lens is capable of and have the switch set to 1 position then press the shutter button. Then change the switch and do the same. Since the lens will be unable to achieve focus, it will go through the maximum range of adjustment, "hunt", and you will see what the difference is.

As Tim stated, the sole purpose is to limit the time spent hunting for focus. But at some point, in setting the switch to limit the hunt time, you will want to use the lens near it's minimum focusing distance and wonder why the heck the camera can't achieve focus lock until you remember about the switch.