I'll try to answer,
Amateur Series scores will apply to all divisons. Same with Semi Pro and others.
I think you can shoot multiple divisions under Amateur if you choose.
The division you shoot in will provide you points via % of winner.
Of the ten 3GN matches they will take your best 3 matches.
You don't have to shoot all 10 and I think you can just shoot 1.
Divisional Series you will gain points per Division, Tac Optics, Iron, Open, etc.
I'm not sure but you may be able to shoot one Division at one match and another Division at the next match.
I spoke to Keith Brown about some of this and I thought he said you could sign up for Amateur or Semi Pro
and Divisional.
However in re-reading the Divisional rules I found this:
1.1: The 3GN Divisional Series is open to any competitor.
However, a competitor may be registered in ONLY one of the following 3GN points series: 3GN Divisional Series, 3GN Semi-Pro Series, or 3GN Amateur Series.
Maybe someone else could confirm rule 1.1 means just that, only one series per shooter.
I'm in under Amateur.
Follow this link and choose your Series, then click "See Complete Rules and Prize Info".
They have updated alot of this as of late.
Hope that helps.
Michael