In most circumstances a unit of troops takes aim and shoots at a single target, obeying the command of their leader who is assumed to direct the unit's fire. The entire unit fires at a single target, such as a unit of enemy troops or a huge monster or chariot.
A unit may divide its fire between two or more enemy targets if it is impossible for all the models to shoot at a single target. For example, it might be that no one 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 shooters.
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 against other more distant targets, even if all the unit could shoot against the single enemy model. This allows a unit to spread its fire against a group of single models.