TRAIT - Troop
A troop is an organized collection of component creatures, typically Small or Medium in size, working as a cohesive whole. A troop is 16 squares in size and has two Hit Point thresholds in their HP entry, under which it reduces in size to 12 squares and then 8 squares. A troop has the Troop Defenses, Form Up, and Troop Movement abilities. Most troops have a weakness to area damage. Because they consist of multiple discrete creatures, they can't be summoned.
Source: Bestiary 3 pg. 310