How does usergroup permissions work?

[riser]

Hey guys, I'll be moving to MyBB(from XF) once 1.8 is out..

I'm just curious how 1.8's User groups work. Here's an example:

Team A usergroup has full access to Forum A but not Forum B
Team B usergroup has full access to Forum B but not Forum A

If I set my primary usergroup to Team A and my additional usergroup to Team B and leave display group to "Primary", I have access to forum A and B.. I would assume that "Yes" overrides "Nos" regardless of order and if primary or not.

If I do what I did above, except I set my display group manually to Team A(which is my primary usergroup).. I only have access to Forum A and not Forum B, eventhough I have Team B in my additional groups.

What am I missing here? Or am I doing something wrong? I've asked an associate, he said that:
"it checks your primary group: 1). if primary group has forum as allowed, permit. end. 2). if primary group has forum disallowed, block. end. 3). if primary group has forum as inherit, then check additional groups. 3a). if one addtional group has forum with allow then permit. 3b). if all additonal groups have forum as inherit or denied, then block."

but I just needed to be sure before moving boards.

Thanks!