Convection Fan Threshold - Min Feed Rate that the Convection Blower will start. YOu really don't need to mess with it too much. Increasing the CFT will cause the blower to come on later (as stove gets hotter). I wouldn't go too much higher, maybe 15-30 (if that.).
SO, if your Feed Rate is at least 8, then the blower will start moving, It goes along with the CFM, which is the MIN. Speed % of the blower. Some blowers don't like to start at slow speeds, so a CFM of 30 would be 30% of full speed as it's min. operating speed.
I know the guys from CoalTrol will chime in for more details.