The enlisted and warrant officer rank are merely reflective on your number of posts. This reflects on the sort of starting character you might have, as, for example, the NH-27 nekovalkyrja is reserved only for the Juni ranks... which pretty much are the top of what normal forum members can get to.
However, Wes gives officer rank promotions to the people whom make significant contributions to the SARP. This is why you see a few people with ranks above enlisted or warrant officer. Wes, as far as I know, is the only person whom can award these kinds of promotions.
For a player, I think the ranking itself indicates how high ranked a newly created character can be if it's not your first one (as in, you have some experience in the SARP)... though I don't think I could just go and create myself an admiral-rank character since the SARP has a big preference for roleplayed history.