From Halopedia, the Halo wiki
Spirits are uniquely identified by their distinctive tuning fork shape. Each "arm" is a troop deployment pod capable of hosting a number of soldiers, while the main cockpit is located at the rear of the ship, and a plasma cannon is mounted below. The space between the two troop bays can be use to carry vehicles and other equipment. Some Spirits are equipped with gravity lifts to deploy troops without risking opening their troop bays.