It appears that Fa is force due to acceleration (F = ma).
Ball nuts are available with internal clearance for low friction, or internal preload for precision motion. Low friction ball nuts are used with low precision (lower cost) ball screws in applications that tolerate location errors of 0.005" or so. Internally preloaded ball nuts are used with precision ground ball screws (higher cost) in applications where higher precision is needed. Typical applications are machine tools where the motion is controlled to +/-0.0001".
Study some ball screw catalogs. You will find specifications for location error, backlash, allowable speed, and allowable acceleration.