“S” is for sport mode, “B” is for braking. I am pretty sure this is a CVT transmission and these settings replace 3rd gear and low gear. S keeps the transmission in lower gear ratios for more power and quicker acceleration. B even lower to assist with braking. The engine does have over rev protection, probably by limiting the fuel at high RPM’s so no harm no foul using these settings. If you are concerned about what they may do to your engine, just do not use them. Keep it in drive and use the brakes.