Resistance basically gives you an idea on how many volts you will need to power your coil (heating element in your
tank). The higher the resistance, the higher volts you will need. A variable volt device allows you to manually adjust your volts according to the resistance of your coil to get the kind of hit you want.
Variable watts is basically variable volts in automatic form. Watts is a calculation between volts and resistance. If you like the hit a 8 watts, you can set your device at 8 watts instead of volts, and your device will adjust the volts to equal 8 watts according to whatever the resistance of your
coils are. So, if you started with a coil with a resistance of 2, and switched to one with a resistance of 2.5, your device will automatically up the volts. If you switched to a resistance of 1.5, your device automatically adjust the volts lower.