This number is the sum of the square roots of the difference of each register content divided by a number to make the result lie between 0 and 0.8. If there is a difference at all, 0.2 is added to the result, to make the the drive towards an exact result stronger. Only registers that have been modified by the program are counted. This is the so-called absolute fitness. The absolute fitness is only used to look at the time progression of fitness.
Relative fitness is used for selection .
The idea is to make the difference in fitness bigger, since the selection pressure would be very weak with a population entirely having similar fitness. The rest of the process can be visualized as each program occupying an area on a wall proportional to the resulting number. We then shoot wildly with a gun against the wall and the programs that are hit are culled. Setting the cull exponent high therefore increases the tendency to only cull bad individuals while a low cull exponent increases the chance of low-fitness individuals surviving . It is imperative that some low-fitness individuals survive to preserve the genetic diversity.