70-200 2.8 Focusing Range Selection Switch?

Can anyone explain to me the "Focusing Range Selection Switch" on my 70-200 2.8L?

Thanks,

Nic

Exactly what it says; it limits the focus range from two different close distances - out to infinity. What it does is keep your lens from hunting across the entire range if you know you'll be shooting at a distance equal to or greater than the minimum noted on the switch. Makes shooting at a distance faster if the camera loses focus.