Sometimes you may find that your models are able to target an enemy unit's flank or rear. This is useful because this can circumvent protection given from shields and other equipment that normally only protect the unit's front.
A shooting unit's position during the Shooting phase determines whether it shoots into the front, flank or rear of the enemy unit. If the shooting unit is in the target's frontal zone it shoots into the front.
As units generally begin the game facing each other, this is the most common situation that will arise. However, if the shooting unit comes from the flank zone, it shoots into the side; if in the rear zone, it shoots into the rear.
If a shooting unit straddles two zones, then the unit is considered to be in the zone where the majority of the models in its front rank are. If there is no clear majority, roll a dice.