I don't know if they have something that officially breaks them down, but I'd imagine its all relative on the general skill level of the area.
If you have an area that is full of a lot of good players, an SL6 could be considered a middle-upper level player, because there are just so many good players at the 8-9 level.
If you are in an area with a lower amount of good players, you'll only have a hand full of 8s and 9s, and 6s would probably end up just being players who might be 4s and 5s in other areas.