The SMOKTech Carto Tank is a hybrid love child of a tank and cartomizer.
Essentially, what has been created here is a cartomizer with a substantially increased capacity. Many users prefer the performance of a cartomizer over standard wick fed tank systems, so this allows the best of both worlds for users with this preference. It was also an ingenuitive way for SMOK to make an effective new tank system without having to redesign something that already works.
How this thing works is as simple as sliding the cartridge into the hole in the middle of the tank, and then filling the tank with liquid through a port on the top of the unit. The liquid travels from the tank to the inside of the cartomizer through small holes in the wall of the carto tube. The cartridge usually comes with at least one hole in the tube, but if the liquid is not being fed into the carto fast enough (either due to heavy vaping or extra thick e-Liquid), then it is possible to punch extra holes using a cartridge punch tool or a small drill bit. Something to remember is not to remove the cartridge from the tank while there is still liquid inside the tank, that is, unless you are looking for a nice e-juice stain on your lap.