In most circumstances, a unit of troops takes aim and shoots at a single target, such as a unit of enemy troops, a huge monster or chariot, obeying the command of their leader who is assumed to direct the unit's fire.
A unit may divide its fire between two or more enemy targets if it is impossible tor all the models to shoot at a single target. For example, it might be that no single target is visible to all shooters either because it is obscured or because it is so close to the unit that it lies outside the arc of vision of some of the shooting models.
In the case of individual enemy models, such as large monsters, Heroes, Goblin Fanatics and similar, it is permitted to divide shots against these and other more distant targets, even if the entire unit could shoot against the single enemy model. This allows shooting unit to spread its fire against a group of single models.