From Halopedia, the Halo wiki
During a game of Attrition, teams compete to kill enemy players to deplete a number of lives (8 lives by default) shared among the members of each team. The team wins a round and scores a point when they have depleted all enemy lives and all enemy players are downed. A team wins when it reaches score limit (2 points by default).
Shared Life Pool
Each team’s shared life pool is indicated on the scoreboard, with additional four icons representing each team member’s status: circle indicates the player is alive, cross indicates the player is dead and awaiting respawn, and downwards arrow indicate that the player is downed and awaiting revive.
Reviving Downed Allies
When the shared lives are depleted, any players killed instead enter a downed state, in which they cannot respawn. Instead, they must be revived to return into play: allies need to interact with a holographic sphere containing the downed player’s Personal AI, located at where the player was downed. If the player’s holographic sphere fell off the map, they cannot be revived.
When reviving allies, players are unable to move or perform other actions during the interaction time (3 seconds by default). Revived players are invulnerable for a short time (1 second by default) after being revived, but they cannot move nor perform other gameplay actions during this time.
If the life pools of both teams are depleted, the danger zone appears at the edge of map. The danger zone gradually moves towards the middle of the map, creating a circular safe zone that shrinks over time. Any players caught in the danger zone take damage to their energy shielding, then health, over time, until they either are downed or exit the danger zone.
When a team has depleted all enemy lives and downed all enemy players, it scores a point. A new round then begins, with both life pools replenished to the maximum number of lives.
By default, Attrition games are 10 minutes long, and the team that first reaches the score limit (2 by default) wins.
Attrition is intended as a 4v4 Arena game mode. Subsequently, it is supported by the following maps:
Unlocks and rewards
The following medals can be unlocked in the Attrition gamemode.