What is the difference between ARSPD_USE and TECS_SYNAIRSPEED?

Hi!,
My aircraft does not have the airspeed sensor and I want to control its air speed. First, I set ARSPD_TYPE=0 because there is no airspeed sensor.
Then which is right to set ARSPD_USE=1 or TECS_SYNAIRSPEED=1?
BTW when I set TECS_SYNAIRSPEED=1, the parameter ARSPD_USE disappears.