Damage modifier

Damage modifiers are ability and talent effects that affect the damage a unit takes or deals by given percentage of its scaled base damage. Damage modifiers can thus be split to offensive and defensive damage modifiers, based on whether they affect the damage dealt by the attacking unit or damage taken by the receiving unit. Damage modifiers may also affect only a certain type of damage, certain Abilities and/or certain type of units, e.g. 'Heroes' or 'Mercenearies', as defined in the effect's description.

All damage modifications are calculated when the target is being hit by the attack.

Some effects, like Percent damage are unaffected by damage modifiers.

Offensive damage modifiers
Offensive damage modifiers affect the attacker's damage output. Most offensive damage modifiers fall under following categories:
 * All-damage buffs and debuffs, e.g. Zul'jin's "Headhunter" and Tyrande's "Harsh Moonlight"
 * Basic Attack and physical damage buffs and debuffs e.g. Executioner
 * Spell Power and other spell damage buffs and debuffs
 * Damage buffs and debuffs affecting certain abilities, e.g. Azmodan's "Tide of Sin"

Defensive damage modifiers
Defensive damage modifiers alter the damage taken by the target of an attack. Almost all defensive damage modifiers in the game are implemented as Armor, reducing or increasing the damage taken by the percentage of Armor the unit has. Chen's "Stagger" and Samuro's "Harsh Winds" are few of the defensive damage modifiers not classified as Armor.

Stacking
Offensive damage modifier most often stack additively, meaning that they do not benefit from each other. However, there are some instances, where the stacking can be multiplicative, like D.Va's "Defense Matrix". The stacking of a certain effect can also be a combination of the two, where the effect stacks additively with itself, but multiplicatively with other offensive damage modifiers.

Defensive damage modifiers consist mostly of Armor effects, which do not stack, but only apply the highest positive and highest negative effect currently active. The ones outside Armor category usually stack multiplicatively with other defensive damage modifiers.

The collective offensive and defensive damage modifiers then stack multiplicatively with each other, as one affects damage dealt and the other damage taken.