[Mod Says]
The reason that there is no explanation on the awards page—and the reason why the previous two answers were moderated—is that the Atlantis awards (aka all of the awards with white plaques) are “secret” awards. You have to either figure them out yourself or hope that someone will be willing to let the secret slip (usually via PM, though, since we discourage public explanations of how to get them).
That said, a lot of people are willing to share their knowledge. And any guesses you have can be sent to one of the mods (who can give you a “yes” or “no” regarding your guess).