Membership classes?

I was a little surprised to find that although UKE has (as far as I know) been running for only a few days it now apparently already has three different classes of Membership :- New Members, Members and Active Members. Purely as a matter of interest (since I have no particular desire to rise from my own humble status) can anyone explain the distinctions between the three classes? Is it based on timescale, number of postings, number of reviews, or is there some other more esoteric attribute that defines one`s position in the hierarchy
Oops! Sorry I spoke, I have suddenly advanced one rung of the ladder between postings, so the mystery is solved. Now I have just to keep posting to see when I shall be considered sufficiently active


Below is screenshot of the admin screen.
That's the default configuration, I've not changed it since setting up the forum.
Might make changes to it later.
Right now it works by postcount, but the 'ladder' can be changed to number of likes a person receives.