Flak is a weapon type found on both the TEC Loyalist and the TEC Rebel titans, Ankylon and Ragnarov-class. Both have four launchers facing forward, and four aft. These launchers send out munitions that detonate some distance away from the target, blanketing it with a cloud of shrapnel rather than a single shell. This allows the Titan to deal damage to strike craft, but is used against any target that gets within range. There are no upgrades for it.
Flak on Wikipedia