In axis and allies revised each AA Gun flown over in the combat move portion gets to shoot at each aircraft that flies over it.
So if you fly 5 fighters from W. Russia over Caucasus (which has an AA Gun) to attack Persia (which has an AA Gun) and then fly 2 bombers and a fighter from Egypt to Persia you would have to roll the following:
1st) the AA Gun in Caucasus would get one shot at each aircraft (and the die rolled is directly related to the fighter, you cannot chose your causalities)
2nd) the AA Gun in Persia would get one shot at each aircraft (again, roll the die individually for each aircraft and bomber, you cannot chose which units die or live.)
I personally, when playing on a board, like to roll the die for each fighter and bomber before placing that fighter or bomber on the battle board. It helps anchor the idea that this AA Gun is effecting THAT unit.