Highglove: The rules of golf only allow you to replace a ball with the same type, not any ball in your bag.
Name the rule no. please! I play by the rules, but I never came across it!
Actually, as Dazza explained, it's not a rule of golf but a term of certain tournaments. Pros have it, and top amateur tournaments may have it, too.
I prefer the "closed bag" idea - it reflects the designers' desire to follow the rules of golf (*), and the programmed limitations.
(*) which they did quite well, with a few exceptions.