This is what the manual says in the introduction:

Control Functions:
Any one of the following 4 control modes can be selected in the parameter settings.

S Position Control (Factory Setting)
Controls the position and speed of the Servomotor very precisely with pulse-train input signals. Any one of the following 3 pulse trains can be selected: forward/reverse pulses, feed pulses/ directional signals, or 90 degree differential phase (A/B phases) signals.