Canon 100-400mm f4.5-5.6l is usm lens operation question

Hello all. I wanted to ask something about the operation of the Canon 100-400mm f4.5-5.6 lens. I'm thinking about renting one to try out so I downloaded the users manual. In the manual I see there is something called the Focusing Distance Range switch. I just wanted to know if anyone could tell me how that is supposed to be used because I read the description in the manual and I'm still unclear on how and when I would use it. Thanks in advance for any input.