A battlegroup formation is a group of UNSC ships that were sent out on missions that were almost always going to result in a battle. The typical formation before the Human-Covenant War consisted of 2 Frigates and usually a Cruiser. But when the UNSC went to war with the Covenant the UNSC discovered it's typical battlegroup was too weak to handle the tougher Covenant ships. So the UNSC started to use either a lot of smaller,less powerfull ships, or a small amount of large warships. But some of the biggest space battles consisted of 300 or more UNSC ships.