newbie question

Hi All
I am trying to learn all about cnc. One question that keeps puzzling me is
when you use Cam software you need to tell the program what kind of cutter
you will be using for it to generate the G code. When you operate the mill
operating program it also asks what type of cutter you are using. My
question is why the redundency. If the G code is based on the cutter why is
the machine software asking for the cutter information too.

Thanks Steve Ross