I guess THC is just a more universal term. Ohmic and voltage are two types of THC, not sure if there's others. I wonder what lasers use?
So your able to cut thin stuff with out using Ohmic and just using the voltage control. Interesting so you just set material hight and run with it.
Lets get back on track, THC and AVHC are one and the same. Ohmic is only used to detect the material on the initial pierce uses electrical contact, after that if cutting in Auto mode THC/AVHC determines the distance from the material. There is also Touch Torque used on older tables to determine where the material is on the initial pierce, this uses a resistance/force value.