Each wheel is the permutation of the letters. There are five permutations provided but you can create your own as long as it is a real permutation (doing so you leave the default executions of the historical Enigmas). The permutation can be offset to increase the number of possible wheels to use (instead of 5 you have 5 × 26).
Additionally you must specify at which letter an overflow to the next wheel will happen.
The offset allows to rotate the permutations relative to the alphabet. With offset 1 no shifting takes place. With offset 2 B will be used instead of A, C instead of B and so on.