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.
Tactical meaning
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
FORMING UNITS
The models that make up your Warhammer army must be organised into ‘units’. A unit usually consists of several models that have banded together, but a single, powerful model such as a lone character, a chariot or a Dragon, a war machine and its crew, and so on, are also considered to be a unit. A unit consists of 1 or more models that are arranged in orthogonal base contact with each other, which is a fancy way of saying ‘edge-to-edge and front corner to front corner’. See the diagrams below – it’s much clearer as a picture than it is in words. All models in a unit must face the same direction. In addition, all models in the unit must be arranged in a formation that consists of one or more horizontal lines, called ranks and a number of vertical lines, called files. This is why we often refer to basic warriors as ‘rank and file’ troops. As far as possible there must be the same number of models in each rank. Where this is not possible it must be the rear rank that has fewer models. Once formed into a unit, the models move and fight as a single entity for the rest of the battle.
Correct and incorrect unit formations:
REMOVING CASUALTIES
Some of the models in a unit (probably quite a lot of them) will die as the battle progresses. When casualties occur, models are removed from the back rank of the unit. If the unit is reduced to a single rank, casualties must be removed evenly from either end of the line.