[Plums]

If the MAF fails, (or you remove the MAF for a MAFLess tune) the PCM will only use the Low Octane table. This is done for safety reasons, perhaps this is what you were refering to.

The SD(Speed Density) enhanced option basically lets the PCM use both High and Low Octane tables when running MAFLess. It also allows you to use Alpha-N tuning (for any part of the VE table) to tame big cams and/or individual throttle body set-ups.