A Formation Fighter is used to fighting alongside his brothers-in-arms in very dense formations; unless they're a Bad Formation Fighter, anyway. A square's maximum amount of Size points is adjusted by the lowest Formation Fighter value in its square. For reference, units without this trait are assumed to have a Formation Fighter value of zero, so even one of them in the square holds the amount of Size points a square can handle at 10; unless a Bad Formation Fighter is present, of course.

The maximum number of units a square can have is 10.

This is the "Bad Formation Fighter" icon, by the way.

• Several nations have troops with Formation Fighter 2. For squares that only contain Formation Fighter 2's, 12 Size points may be accommodated; that's six Size-2's, four Size-3's, three Size-4's, or two Size-6's, but still only ten Size-1's.
• Arcoscephale has Size-3 troops with Formation Fighter 5, and it can fit five of those guys into a single square.
• Therodos has Size-3 troops with Formation Fighter -2, or "Bad Formation Fighter (-2)", so having even one of these guys in a square will limit the number of Size-3 troops the square can handle to two.