At launch, it includes a first-person shooter battle royale that comes in three types: solo for 100 players, teams of three for 102 players and a 250 versus 250 player mode called Massive Clash. It's all set on a huge 8k x 8k, four square kilometer map called Echoes of Amerish, which is based on Amerish, the third continent added to the world of Auraxis for PlanetSide 2.
While Arena launches as a battle royale experience, Daybreak does not intend for it to be a battle royale game. New modes like Capture the Flag, Team Deathmatch, Search and Destroy and Global Conquest will be introduced each new game season.
I have a feeling the 250 vs 250 mode is nothing more than conquest.