What is Normal Resistance, Low Resistance and High Resistance? what suitable voltage to different resistance?

To understand Standard(Normal) Resistance(SR),Low Resistance(LR) and High Resistance(HR/HV) of the e-cig atomizer/cartomizer/clearomizer/glassomizer, it helps to be familiar with Ohms Law.  
 
*Power (measured in watts) is the intensity of the vape. Unit of electrical energy or power - one watt is the product of one ampere and one volt - one ampere of current flowing under the force of one volt gives one watt of energy. 4-8 watts is the “sweet spot” for most vapers.
• Watts = Volts X Volts / Ohms

*Current (measured in amps) is what can burn out cartomizers. one ampere is the current which one volt can send through a resistance of one ohm. Roughly speaking: around 1.5 amps is fine; 2.0+ amps is risky.
•Amperes = Volts / Ohms

But watts and amps are not properties of cartomizers or batteries. They are derived from cartomizer resistance (measured in ohms) and battery voltage (measured, of course, in volts).

•Voltage - unit of electrical potential or motive force - potential is required to send one ampere of current through one ohm of resistance, you can get this parameter from the battery.
•Ohms - unit of resistance - one ohm is the resistance offered to the passage of one ampere when impelled by one volt, you can get this parameter from the e-cig atomizer (or cartomizer, clearomizer, glassomizer).  Low resistance generally refers to 1.8 Ohms-2.4 Ohms, the normal resistance refers to 2.8-3.2 Ohms, and High resistance refers 4.0+ Ohms.

So we need to balance battery voltage with cartomizer resistance to get an ideal vape intensity (4-8 watts or so) without burning out the cartomizer. If the voltage is too low and/or the cartomizer resistance is too high (relative to each other), the watts are low and you get a poor vape (little throat hit, vapor, and flavor). On the other hand, if the voltage is too high and/or the cartomizer resistance is too low, the amps are high and you can burn out the cartomizer.

If Watts< 4.0, too low to power the coils effectively but will not hurt to try it.

If 4.0<Watts< 8.0, Safe to vape, can give you decent vape.

If Watts>8.0, May damage or burn out your coils, not recommended.

Enter any two known values and press "Calculate" to solve for the others. For example, an eGo havng a rating of 3.7 volts AC and using a carto of 2.5ohms will draw 1.48 Amps and 5.476 watts. In normally, you can know the Voltage from the battery and the resistance from the cartomizer.

Voltage (E) = Current (I) * Resistance (R)
Power (watts) = Current Squared (I^2) * Resistance (R)
Power = I*E = E^2 / R