What do all the numbers and letters mean on the back of the 5 series?

First number - Series (in this case, 5). I am going to assume you mean e39 here. Second & third number - size of engine (i.e. 525i = 2.5L, 530i = 3.0L, 540i = 4.0/4.4L) Letters - i = injected, L = luxury (usually long wheel base), d = diesel.