Nope but good guess. Back on Bungie in like 2008 and earlier, I think, they had a ranking system for members, based on activity in the forums. The main 3 ranks were heroic, legendary, and mythic. But around '08 and '09, they took those ranks out. Now it's impossible to get the ranks, but those who had the ranks, have access to the symbols, according to what rank they had.