Some vehicles, particularly battle tanks, carry a hefty arsenal of weapons. When a vehicle fires, it uses its own BS characteristic (representing the BS of the crew) and shoots like other units - all its weapons must fire at a single target unit. Although vehicles can fire light weapons on the move, they are limited in the number of larger weapons that they can fire.
Vehicle Shooting
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 38