So I assume that would be an example of getDamageToTargetShieldsMult().
Graviton is a damage
taken modifier, applied to the target ship. getDamageToTargetShieldsMult() is a damage
dealt modifier, applied to the firing ship. Imagine a hypothetical "deal 10% more damage to shields" officer skill, that would be it. I think nothing in vanilla currently uses it, though.
As already explained by float, damage taken modifiers are never additive with damage dealt modifiers, and have several "partitions" among them as well.
EDIT: On top of that, there are also combat listeners that can modify damage. Vanilla uses them sparingly, mostly for "bonus X damage against X" effects found on Squall / Mining Blaster / Breach. Even these are all different. Squall is a modifyDamageDealt listener. Honestly not sure how it interacts with other bonuses, but my guess is that it's affected by damage taken modifiers, but not damage dealt ones.
Breach / Mining Blaster / Disintegrator have their own dealArmorDamage functions, with full-on spaghetty manual application of damage modifiers. Hull size based and armor taken modifiers are applied, all others are ignored. So bonus armor damage from mining blaster, for example, should be affected by TA skill, NOT affected by energy mastery, and affected by entropy amplifier.