Flak was designed by the GTVA some time after the end of the Great War. Oddly enough, the Shivans seem to have put this technology into service at about the same time. Flak is more effective against fighters and bombers than warships. However, flak guns are also effective against other smaller-sized ships such as transports, freighters, or even weaker cruisers. When a projectile of flak explodes, it has a small damage radius.
Flak projectiles usually do more damage on hull plating than on shields. Flak guns are also known to be nearly useless against subsystems.