On the battlefield troops don't fight as a disorganised mob of individuals, they fight in properly organised units with officers to lead them. Units march in disciplined ranks, and they are trained to manoeuvre as a solid block.

Generally speaking, a unit is made up of troops of the same type. For example, it may be a unit of Men armed with crossbows and wearing light armour. All the models in a unit will be of the same race, and will have the same weapons and armour. Although it's true that creatures of the same race vary - some are weaker, faster or braver than others, we assume that the differences average out over the unit as a whole.

Units are arranged on the battlefield in a formation which consists of one, two, three, or more ranks as shown below. Units can change their formation during the battle, but this takes up time and may slow the unit down.