I think (I hope, if I understood correctly) that Decurion was asking about the Points Cost (Roster Size) being set when you choose which Rules you select.
As an example, Games Workshop's School League rules specify that the points limit is 600. Army Builder allows you to specify any limit you want, including 'Unlimited', even after choosing School League as your Rule.
It would be handy if the Points Cost was automatically set to 600 when the School League Rule is selected.
(Decurion, I hope I've understood your original post correctly)