It means that I know the throat horn assembly is at exactly the right angle.
It suprised me how out of line the top/btm can be if I just relied on the angle of the cutting blade to be right (I know bills design allows for descrepencies).
So far the sides will only need to be removed when installing the rear throat halves & side braces. I figured simply cutting the sides out first and using these gives a very accurate end box size.
Certainly worked for me

Dave