@jinchengde this will probably be very common for heli users because the tail rotor thrust will change based on collective so the amount of yaw input required changes whether you are in a hover or in forward flight.
As @iampete points out, we are still determining what a good threshold might be. I think for heli users, it could point out which users have not mechanically trimmed the tail rotor for hover. If you are an experienced arducopter heli user, then you may choose to just disable the warning by setting bit 1 to zero for the FLIGHT_OPTIONS parameter.