Boarding Action
From Halopedia, the Halo wiki
Template:Multiplayer Map Infobox
Boarding Action is a multiplayer map in Halo: Combat Evolved. There are two parallel human battleships, possibly UNSC Prowlers. In addition, there are 4 teleporters on both ships, two on the first level, and two on the fifth level, which are the only means of transportation between the two ships. This map is usually picked by hosts for its extremely exciting sniper battles. Other hosts may turn off sniping to force players to board the other ship. This usually results in a frenzy of explosions near each teleporter as each team frantically tries to stop the other team on their ship. The ships are not in fact that far apart - it is simple to throw a grenade to the other ship.
An interesting way to simulate ship-to-ship combat is to do a 4-Xbox system link, and have 16 players, eight to a team, loaded with all Rocket Launchers. Volleys of rockets across the two ships greatly resemble hot exchanges of cannon fire between two eighteenth-century ships-of-the-line. It is not at all uncommon in any game type to find yourself standing directly above or below an enemy as the ships have several floors, with each floor number marked on the wall. Often times one player will come through a teleporter right in to another player without much warning. Should you fall into the space between the two ships, obviously you will die, but after falling a certain distance, you hit an invisible floor, for lack of a better term.
Theories
Bungie has, in the past, stated that the Multiplayer levels in Halo: Combat Evolved are canon to the story. This may seem strange at first, as the Pillar of Autumn was thought to be the only ship in-system during the events of Halo.
So it is only natural for one to assume that these two smaller ships are UNSC Prowlers, ONI Stealth ships, at Halo prior to the events of the first game, possibly the ones setting up the Multiplayer maps Wizard, Rat Race and Chiron-TL34, the SPARTAN clone training complex, all human-made training grounds.
Why are these ships so special? The crew of the Pillar of Autumn crash landed, they would not have had sufficient supplies to construct these -- Nor the need to construct them, as they were not recruiting any new troops.