Following on from John's post, once the target CG envelope has been CHOSEN by the designer(s) and the various systems and components all designed so as to meet that design goal, you then have to go and test the blessed thing and find out if all your guesses - err, detailed design calcs
- were right or not.
That's where all those strange cut-outs and kinks in envelopes often come from - if it's not a nice neat shape, chances are there's an 'oops' hiding somewhere.