I've always agreed with this following definition, except for the part about A players and Open player always stringing racks together. It depends entirely on how the table is breaking, and how good a breaker the player is.
D- Player
>will not run a rack
>average run is about 3 balls
>with ball in hand, will get out from the 7, one out of 3 times
>rarely plays a successful safe
C-Player
>will probably run one rack, but usually not more than one
>avg. run is 3 to 5 balls
>with ball in hand, will get out from the 7, two out of 3 times
>mixed results when playing safe
>inning ends due to botched position, missed shot or attempting a safe.
B-Player
>Able to run 1 to 3 racks
>avg. run is 5-7 balls
>with ball in hand will get out form the 5, 2 out of 3 times
>most of the time a "B" player will play a "safety" which maybe hit easily 2 out of 3 times
>a typical inning will end with a missed shot, a fair safety, or a won game
A-Player
>will string 2 to 3 racks
>avg. ball run, 7-9
>with ball in hand, will be out from the 3 ball, 2 out of 3 times
>typical inning will end with a well executed safety or a win.
OPEN-Players
>average 8+ balls
>string racks together more than once in a match
>is a threat to run out from every ball, from every position, every inning
>typical inning will end in excellent safety or win