Well based on my knowledge --it was been done by selection by the admin either cyrus or theymos, once you pointed out it means you are very active on that board and as they said above the criteria should be you are a good reporter. That will perhaps be temporary, just like the moderator on our local Philippine board, he is no longer a moderator now. However, regarding moderators -- I think most of them are global moderator which can able to moderate on any board.