From ORCA North America:
A new version of RT motor in 4.5T with smoother handling than ORCA’s last released RT 4.5T. RT-S 4.5T (“S” stands for Smooth) is not a replacement but additional model designed for drivers who are looking for ORCA Factory Special Tuning delivering smoother car handling and control. Recommended also for small to medium size tracks with low to medium surface traction. Comes with 12.1mm Rotor.