Hi @FRANKIEDABRUSH- first are you actually looking at a complete price list or are you looking at the Print Book? The NPP Exhaust is a dual mode system that increases horsepower and results in a more aggressive exhaust sound, along with larger 4" exhaust tips versus the standard exhaust. I believe you activate it via a switch. The FE4 is the latest magnetic ride system that is an option on the Z51 packages.I'm looking over options on the price list. What exactly does NPP-EXHAUST SYSTEM PERFORMANCE and FE4- MAGNETIC SELECTIVE RIDE CONTROL w/PTM mean? THANKS.