All models in a unit have to shoot at the same target. Due to the relative positions of the shooting unit and the target, it might happen that some models in a firing unit can shoot the target while other models in the unit cannot (the target may be out of their forward arc or completely obscured by terrain or other models). Sometimes, some models in the shooting unit will be out of range. In these situations, you're free to continue with the shot, but only those models permitted to shoot the target (and that have it within their weapons' range) will fire.