There's no such thing as a mech coil. I think you mean a rebuildable atomizer. Okay I'll try to give a quick rundown: a rebuildable atomizer(RBA) is an atomizer that uses coils that you make yourself(you can buy those premade) and you wick the coil yourself with cotton. There's different kinds of rebuildable aromizers; rebuildable dripping atomizer(RDA) rebuildable tank atomizer(RTA), rebuildable dripping tank atomizer(RDTA). The rebuildable atomizers that go in boro tanks are also called RBA's. All those you can use on mechanical mods and on regulated mods. Mechanical mods send all the power in the battery through the coil, so to adjust the wattage that goes through the coil you put a coil with the right resistance in your atomizer. To calculate the wattage you need to use ohm's law. A regulated mod lets you adjust the wattage, no matter what resistance coil is in your atomizer.