From Halopedia, the Halo wiki
Battlegroups are subdivisions of a UNSC fleet, flexible fighting formations that can formed on an as-needed basis in several different tactical formations, which themselves can be "plugged together" into ad-hoc task forces. Battle groups are usually created on a temporary basis, with a specific mission or to accomplish a specific task. Battle groups can range in size from as little as three ships to larger carrier groups with smaller "tactical" carriers at their core. These battle groups are referred to as carrier groups.
Under the NAVCOM rank structure, battle groups are usually commanded by a rear admiral, though there have been instances of smaller battle groups being led by captains. Each battle group is assigned a number or a code name.
The concept of a flight may be related to the concept of battle groups.
Known UNSC battle groups
Although the Covenant organize smaller fleet units on a temporary basis similar to UNSC battle groups, their naval doctrine identifies such units as fleets rather than battle groups; for example, the Fleet of Sacred Consecration was still identified as a fleet despite only consisting of 15 ships. Instead, the term "battle group" is applied to certain tactical formations of Covenant ships that are part of a larger fleet; for example, the Fleet of Particular Justice had several "CCS-class battle groups", each consisting of three Ket-pattern battlecruisers. Similar to the naming of their ships and fleets, the Covenant themselves often derived the naming of their battle groups from their religious nomenclature.
- Halo: Combat Evolved Anniversary, Library: Ket-pattern battlecruiser
- Halo: Fleet Battles, Cole Rulebook - page 9
- Halo: Ghosts of Onyx
- Halo Wars: Genesis
- Halo: Evolutions, "The Impossible Life and Possible Death of Preston J. Cole"
- Halo: Warfleet, p.40-41
- Bungie.net: Military Power: Reach (excerpt from CAA FACTBOOK l.update 1.5.2550)
- Halo: Official Spartan Field Manual, PART 8: UNSC ORGANISATION, NAVCOM Ranks (p. 140-141)
- Halo Graphic Novel, page 86
- Halo: Ghosts of Onyx, page 329
- Halo: The Essential Visual Guide, page 180
- Halo: Fleet Battles, Core Rulebook - pages 120-121
- Halo: Combat Evolved campaign level, The Pillar of Autumn ("I've isolated approach signatures from multiple Ket-pattern battle groups, make it three capital ships per group.")
- Halo: Ghosts of Onyx, page 45