Boarding Action (HAHAHAHAHHA) is a level in space where a ship has been sliced with a Covenant butter laser. All the Marines and UNSC personnel have died from shock and lack of oxygen, but mostly because Bungie says so. The only survivors are Spartans who were former friends, but are killing each other with their last breaths to reach oxygen tanks that provide enough oxygen to go to another place that has oxygen, etc.
The ship is in a isolated area in space where no screams can be heard. Some Halopedians suggest that the divided ship is not 1 ship, but 2 Halcyon-class Cruisers and that "they may have been most likely amusement parks made by the UNSC". But, everyone knows that when space cruisers cruise alone in space means that they are wasting time to make up a heroic story to tell their officers so they can get promotions. So, that suggestion has been terminated.