I think it is a Forum metric, not a personal one. I think it is automatically assigned based upon tenure on the Forum, number of posts, number of likes, etc. But one of the admins can shed more light on it.
Maybe that should be changed, I have been meaning to donate and become a supporter, didnt realize I would lose my EF-5 rating if I do ! LOL Not that it would stop me from donating, but might be a small positive change to consider.