Enemies of the Tyranids have learned many valuable lessons at a terrible cost in lives lost and bloodshed. One of the most important stratagems has been teaching troops to evaluate the threat represented by different creatures and pick the right targets within the swarm even in the heat of battle.
Due to this, an opposing player's line of fire is only blocked by terrain, vehicles and models in close combat. Other than this he can freely choose to direct fire from his units at any Tyranid brood that is within range during the Shooting phase.