The p is the wattage setting. The difference between watts and volts settings is that on volts it will cositently put out the power level you select. With the wattage setting it will adjust itself to the resisence of your coil. So, say you have a 1.8 at 7 wattss, then change to a 2.2 ohm coil. It will automatically raise the output to accomodate the added resitence.(in theory) With the voltage setting you will have to raise the output manually. If you are going to be using one setup all of the time as far as a coil, then either will work. The wattage is more geared towards folks that change there setup quite a bit. I like mine set at 7 watts for what I vape, but as mentioned before, you just need to dial it in for your tastes. Good luck!