Advertising. A RR tank is drilled and has an overflow (typically). A drilled tank may not be set up for an overflow, I guess. Some people who drill tanks with tempered bottom glass, of course, have to drill the walls of the tank, as you can't drill tempered glass (well, not without breaking it, anyway). I usually think of RR as having the holes drilled in the bottom with an overflow surrounding the holes, but I'd ask the person exactly what they mean before buying. HTH