Break Damage is the damage a unit deals after they successfully deplete a target's Break meter, and this is primarily scaled off of the Break Intensity stat. Now, the game's description of how Break Intensity works is a little confusing, but essentially, the Break Damage you deal scales off the Break Intensity of the entire team. It doesn't matter who applies the hit that depletes the target's Break meter. What matters is how much Break Intensity all four of your team members have. Additionally, Break Damage scales with your target's max HP, which is why you should always try to deplete those