From Destinypedia, the Destiny wiki
Combined Arms is a Crucible match type.
Combined Arms is a 6v6 Crucible playlist mode where teams have access to vehicles. Players battle on much larger maps than normal and are able to use Pikes, Heavy Pikes, Interceptors, Heavy Turrets and their own Sparrows in battle. Special medals are awarded for killing enemies with vehicles. Teams have to take control of three flags scattered across a map, and hold them against enemy attack. Points are scored for holding a flag zone, and scoring kills.
Map navigation is important, and use of vehicles to move is a significant advantage. Two points minimum are required in order for a team to have a strong point flow to stay in the lead. Novice players and teams will assume that holding one point and out killing the other team will suffice, but this is rarely if ever the case. Trying to hold three points if often considered a poor idea, as it leads to the team being stretched thin, and leaves all three points open to attack. Heavy Ammo spawns in many different points across the large map, so teams can secure more than one crate each with little difficulty. Heavy Ammo is very strong against turrets and vehicles.