From Halopedia, the Halo wiki
A pack is a small tribal and military group of Jiralhanae led by a pack leader, usually a higher-tier Captain or Chieftain. Each member of a pack is hand-picked by its leader. Packs are organized under Master-packs.
After the Sangheili left the Covenant in Great Schism and the Jiralhanae assumed their former places, the packs traditional to the Jiralhanae's tribal society became the most prominent infantry unit in addition to the lances, now led by lower-ranked Jiralhanae.
Whereas Jiralhanae up to the Captain ranks lead lances comprised of other species, packs are all-Jiralhanae units led almost exclusively by higher-ranked Captains. These are mostly comprised of Minors and lower-ranked Captains, and are usually considerably larger than a mixed-race pack.
At the highest echelons of the Jiralhanae pack structure are smaller packs led by Chieftains. These are usually comprised of Bodyguards and/or Stalkers, and may include lesser races, such as Ultra Grunts.
The Jiralhanae assigned to piloting Covenant aerospace craft such as the Seraph often formed close-knit packs, carving their clan sigils into the hulls of their war machines and fighting, eating and dying alongside one another.
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