+% Skill Damage applies a percentage increase to any relevant skill damage (after any synergy bonus), or integer or percentage fire damage added to the melee attacks of the user (it does not apply to physical attack damage converted to the relevant type).
- When present on two weapons, the percentages of both are summed and applied to skill damage, and attack damage of either weapon when using a melee attack.
- It applies to skill damage applied by missiles when created, and other damage when applied. If there is a delay between skill usage and damage or skill usage and missile creation, swapping weapons or other items to apply a higher percentage can result in more damage. 
- Character screen displays it being applied to ranged attack damage, but this is not actually the case. 
- It does not apply to a missile's own damage, with the sole exception of Meteor pyres (since these are specially flagged to apply Mastery). Character screen and skill descriptions may apply it to displayed damage, but this is not actually the case.
- That equipped by the user does not apply to pets summoned by that user (with the sole exception of Hydra); this includes sentry traps, despite what is displayed by the character screen and skill descriptions.