Forge

Forge mode
The forge mode is where a player can spawn objects and create bunkers, checkpoints, spawn points, and even entire maps. Halo 3 was the game that introduced the Forge engine, but unfortunately it was so basic that a small movement would cause the objct to move in spasmatic way. The way players used it was something Bungie had not even considered players could do. It was originally was for placing objects like weapons or spawns on the map. Halo Reach built heavily on the idea, even going so far on the idea that a map known as Forge World was created for this.

Common uses
Many people use this option to create new maps, and Halo Reach had just amplified this ability greatly by adding two more physics, fixed and phased. Fixed allows you to place objects anywhere they cango, even in the air, making objects like supply cases and barriers like they were in Halo Combat Evolved. Phase is able to put objects inside of the landscape and other items.

Options and teams
Objects can have they're colors changed, with a few exceptions, like covenant and UNSC barricades, and every vehicle except the Mongoose.