by voovits » Tue Jan 25, 2011 9:26 pm
the number after the C is the number of outs a pitcher can record before being susceptible to immediate fatigue.
In other words, if an F2 gets 2 batters out, then gives up a hit to the 3rd batter, he is now immediately fatigued. meanwhile, in that same scenario, the F6 (or an F4, and F5 for that matter) would not suffer any fatigue.
It is that reason why most consider F3 to be the minimum safe number to have as their closer.