Assault is a game where the objective is to deliver a bomb into the enemy's base. The player can choose from 9 different multiplayer gametypes: Assault, Capture the Flag, King of the Hill, Oddball, Juggernaut, Slayer, Territories, VIP, and Infection. The player obtains one point for each win in the online and loses two points for each game he quits. The game uses the TrueSkill ranking system for its online matchmaking facilities which is based on skill and experience that is determined by Rating Points. The game also supports a combination of LAN, Xbox Live or split screen. In the Multiplayer mode, up to four people can participate in multiplayer match on a single Xbox 360 in Halo 3 via a split screen layout and up to sixteen may participate in a single match over a LAN or Xbox Live. Each player has identical abilities, but the Arbiter and his Elites have different starting weapons from Master Chief. For plot reasons, instead of having each player be an identical Spartan as in previous Halo games, the first player plays as Master Chief, the second player plays as the Arbiter and the other two players play as Covenant Elites named N'tho 'Sraom and Usze 'Taham, each with their own backstories. Originally Bungie confirmed Co-Op play on a system with two player split screen. The Cooperative play of Halo 3 supports up to four player. Players are awarded gamerscore points for successfully reaching a certain score in each level, and medals are awarded for specific accomplishments. In co-op mode, players can lose points for accidentally committing suicide, dying, or killing a cooperative team-mate.

Points are obtained defeating enemies, with extra points being awarded for stylish moves such as headshots, finding and activating Skulls, with multipliers given for progressing through a level quickly. There is also a campaign scoring mode called the 'meta-game', in which a player is awarded a numerical score based on their performance while completing a level in the campaign. Accessing these terminals offers cryptic text messages from within the Halo Universe. Hidden throughout the campaign levels are several 'Marathon Terminals,' named after the plot-driving terminals found throughout the Marathon campaign. In the campaign mode the player assumes the role of Master Chief. Halo 3 contains three modes of play Campaign, Co-op, and Multi-player. A player can only carry one equipment item at a time and equipment items can be destroyed on the battlefield by gunfire and explosives. The equipment is a new item class in Halo 3, items have various effects and functions ranging from defensive effects, objects which blind or confuse the enemy, or items that can actually harm and defeat. There are four support weapons the machine gun turret, the plasma turret, the missile pod, and a Flamethrower. The player can rip the weapons off of the mount and use them, but he must give up some combat options and his speed will be reduced significantly. Support weapons are often weapons mounted on an emplacement with heavier firepower and unlimited ammo. Halo 3 introduces 'support weapons' and new equipment to the Halo series. Each type of enemy exhibits a distinct style. Enemy AI also has non-combat behaviours to make them feel more alive and realistic. If a player takes a secondary weapon the secondary weapon will be stored and shown on the character's back (for larger, two-handed weapons) or on a leg holster (for smaller, one-handed weapons) making it impossible to hide a powerful weapon to surprise enemies. Most of the weapons are from the two previous games of the Halo series, Halo: Combat Evolved and Halo 2. Players also have the ability to dual-wield, where he forgoes grenades for a pair of weapons. Halo 3 is a first person shooter game which contains elements of vehicular combat and melee combat.

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