There should not be any gaps in first ranks, thus when a model from first ranks dies it is replaced by any model from the rear ranks.
Formation behavior in Dark Omen
All models in the unit must be perfectly aligned in base contact with each other and face the same direction.
The maximum/minimum number of ranks and files that a unit may have depends on number of models in unit. For more details pls. see table.
There are 2 units formation limit:
Tightest formation. Can be set by pressing +
Widest formation. Can be set by pressing -
Unit always auto-reforms to Widest formation as soon as it receives attack order.
units in Tightest formation
units in Widest formation
There should not be any gaps in first ranks and when a model from first ranks dies it is replaced by any model from the rear ranks.
Formation in combat
Both in DO and TT only first ranks of unit may shoot with ranged weapons. For bows it is first two ranks. For crossbows and/or firearms only first rank. Thus, widest formation for ranged unit is the most efficient one.
Since there is Line of sight mechanic in DO, size of unit formation affects unit stealth. Tightest and the most compact formation is more suitable for hiding behind large furns (meshes) such as buildings or hills.
Based on previous in-game observations, formation doesn't give any bonuses in melee, thus has no other tactical meaning.
In TT rules unit with multiple full ranks receives bonus during combat round score calculation