Cartomizers are what holds the juice and the heating element that vaporizes the juice. To use a cartomizer, you fill it until it can't absorb any more juice. Then, as you're vaping, every 10-20 minutes or so, you drip a few drops into the top of it, through a "drip tip". That way, it never gets too dry. If your cartomizer dries out, the filler material that holds the juice will burn or scorch and taste nasty. This will ruin the cartomizer.
With a tank, there is a hole in the cartomizer. You fill the cartomizer as usual, then you put it in the tank, through the bottom hole. Before it goes all the way through the top hole of the tank, you fill the tank 3/4 of so full of juice. Then you stick the cartomizer up through the top hole of the tank. You stick a drip tip in the cartomizer and screw it onto your Lavatube. As you vape, the juice in the tank will seep into the hole in the cartomizer, keeping it wet. This can be tricky because sometimes the hole is too small and won't let enough juice into the cartomizer fast enough. That usually happens with thicker, high VG juice. The cure is to punch a second hole, or enlarge the existing hole to increase the flow into the cartomizer from the tank.
XL cartomizers are longer than standard ones. They hold more juice. They will fit all the way through the tank, but unless tehy already have holes in them, you'll need to punch your own holes.
My advice would be to get a pack or two of standard length, (
Crystal Clear Vaping) or XL length (
Crystal Clear Vaping) 3ohm cartomizers to use outside of the tank. Just use these all by themselves without the tank. A pack consists of 5 cartomizers. They are disposable, but refillable. You will want a different one for each flavor. In other words, don't mix flavors in the same cartomizer.
For the tank, get a pack of pre-punched XL length cartomizers. You can get 1.7ohm Dual Coil cartomizers like the one that comes with your tank. Or, you can just get standard resistance 3ohm single coils if they have them in XL length and already pre-punched. You really don't need low resistance cartomizers with a variable voltage PV like the Lavatube.
The page with all the cartomizers is here:
Crystal Clear Vaping
Whether you get XL or regular length is optional except for the tank where you need the XLs. You really don't need 1.7 or 1.5ohm cartos because you can always just increase your voltage. If you have to get 1.5 or 1.7 because those are the only ones avialable pre-punched for a tank, that's o.k. You can use them. Choose 1.7ohms in that case.