LR = low resistance. SR = standard resistance. HR = high resistance (for variable voltage mods).
S = standard length carto. XL = extra long length carto.
Cartomizers all have the same diameter to fit into any cartotank. There are standard (35mm) and XL length (45mm) cartos for standard and XL length tanks. The resistance (ohm rating) of the coils inside the carto affect how hot the vapor will be.
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Dual coil vs single coil is a personal preference. You can put a single coil carto in a dual carto tank, and vice versa.
I prefer single coil because I found that it is hard to keep dual coils from burning the top coil's filler due to their design. The top coil is about mid-shaft in position inside the carto, which is higher than wick holes in a punched carto (see below diagram - red arrows represent juice flow). Even if you properly prime the carto, the juice from the tank will not consistantly keep the top coil wet enough to keep it from burning the filler.
Placing a hand-punched hole higher in the carto shaft is not the answer, because when the juice level in the tank falls below the higher hole, air gets sucked into the cartomizer and causes dry hits. Enlarging the bottom holes may result in flooding the carto.
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Single coil cutaway (click to enlarge)
The name "DCT" or dual coil tank is a misnomer. This is what Smoktech called one of the first cartotanks made, and included their new dual coil cartomizers. The name just stuck on to mean all types of cartotanks (even those made by other tank manufacturers), much like how Bandaids is synonomous with adhesive bandages and Kleenex is synonomous with facial tissues, etc. Many novices believe only dual coil cartos can go into a DCT, which is incorrect.