The step from FS B to FS C is quite big as it requires more complicated modelling of aerodynamic and ground handling effects
With the exception of autorotational landings to a touchdown, an FFSB/FTD3 is equivalent to an FFSD with regards to matching reference data. The same objective QTG tests are required and they must pass the same tolerance criteria. Inspectors might also opt to evaluate autorotational landings to a touchdown for a FFSB/FTD3 device against the FFSD tolerances, which was the case on the Bristow S92.
Essentially, it's a device with FFSD aero/rotor/flight-controls/ground-reactions/engine/AFCS modeling.
As previously mentioned, the difference between the Bristow FFSB/FTD3 and a FFSD device is with regards to motion, sounds, and visuals.