What is the difference between RFC-A and RFC-S mode on my Unidrive M700?

RFC-A mode stands for "Rotor Flux Control Asynchronous" mode, and is used to control AC induction motors.  Using RFC-A mode in "sensorless" control means that the drive is reading the rotor speed from the EMF on the rotor itself to do closed loop control.  Using RFC-A mode in "closed loop" or "vector control" typically means that the the drive is using a physical speed feedback device to perform closed loop control.

RFC-S mode stands for "Rotor Flux Control Synchronous" mode, and is used to control AC servo motors with permanent magnet rotors (power factor of "1").  Using RFC-S mode in "sensorless" control means that the drive is reading the rotor speed from the EMF on the magnets on the rotor itself to do closed loop control.  Using RFC-S mode in "closed loop"  means that the the drive is using a physical speed feedback device to perform closed loop control, along with hall sensors to determine proper commutation.