What exactly does "Automatically set as primary group" do?

[dax (Daniela)]

@tophee

So if I combine 1. and 2. that means that if I activate “Automatically set as primary group” for multiple existing groups, the order in which I do that matters (because the last one overwrites all previous ones (in case of overlapping membership), right?

Yes, in my case I have a single user in 2 primary groups. When I set the 2nd group as primary in his user setting the last group (2nd) became the primary