[Plugin: Subscribe2] What does the check mean?

I have subscribers with R (registered?) and some with a Check before their name. the Checked ones also have the 3 boxes in front. (And what does ! mean?). I only got the boxes (and a check) to show up by double registering the email address. If i delete one of the double entries it goes back to no boxes and an R. And the subscriber count display on the site goes down, meaning i could take it to zero and still have many subscribers. PS – i have seen so many plugins with so little basic documentation, even in the how-to txt or pfd.

https://www.ads-software.com/extend/plugins/subscribe2/