No matter how hard the manfacture tries sometime the wings (or vertical stabilizer) are not installed exactly right. So the aircraft may require a small amount of trim to fly stright and level.
To accomplish this during a test flight the aircraft is trimed to fly stright and level and the required amount of trim rerquired is noted. After landing, the recorded amount of required trim is put into the system. The trim unit is then removed and reclocked to zero.
This is call "trim bias", even though the trim wheel reads zero there is a small amount of trim always there and the aircraft flys stright and level.