mischa I've attached an image of a Lamp Module. If you open the image [below] you will clearly see the wheels or dials on the module that are used to set the codes. The House Code is the letter (A-P)... and the Unit is the number (1-16)... hense you have a total of 256 codes.
Most modules and devices are shipped with the default code of A1. I think most users select other codes based on a system that makes sense to them and would be easy for them to remember. Often people use B for Bedrooms, L for the Livingroom, C for Cameras... and so on.