Also some rebuildables, both tanks and drippers, have insulators that you don't want the hot coils touching. By adjusting the coil up or down you can make sure the insulator isn't burning.
Parallel coils are when you wrap two wires at the same time, each coil having a coil from the other wire next to it. This allows you to build at a lower resistance while maintaining lesser ramp up time, and the increased surface area increases vapor production. Using thicker wire also increases surface area, but it will take more power to heat up the thicker wire, and it will cool slower as well.
-Treeburner