Earn Elo: count the ELO you EARN during the event time (don't count the ELO you lost for being killed)
=> Encourage players to PLAY rank match as much as possible
=> So you can setup it with/without leaderboard on your own purpose
Reach Elo: count the total Elo you have at the end time.
=> Without Leaderboard: Reward players when the total Elo reach some minestone.
=> With Leaderboard: Reward players with highest total Elo at the end time. A.K.A it's an auto rewarding system for a Ranking period