Let me take a shot at this, although I may get it wrong.
In Western 3 Gun, the shooter is committed at the buzzer. When that beep goes off, it is up to the shooter, no matter what he is signed up as, to shoot the stage as is.
The difference between Open and Amateur comes up in coaching. For the Open category there is no coaching, not from the timer and not from the crowd. The Open shooter is on his own. The Amateur shooter does get the advantage of coaching from the timer. So if you are Open and can not remember if that target needs 3 or 5 shots, do not ask the timer, just give it your best shot

Did I get it right Sammy?
Thanks Howdy, great question.
Best Regards,
Black Jack Traven