715. Bonus Damage applies to the total damage Dealt by one instance of the action.
715.1. If the Deal action has a single target, the amount of Damage to that target will be increased by the Bonus Damage granted to it.
715.2. If the Deal action has multiple targets, the amount of Damage dealt to each target is increased by Bonus Damage individually and separately. Example: Singularity is a spell that says "Deal 6 to each of up to two units." A player plays Singularity while they also control Annie, Fiery, a unit that says "Your spells and abilities deal 1 Bonus Damage." Singularity deals 1 Bonus Damage to both of its targets, dealing 7 to each.
715.3. If the Deal action Splits damage, then the Bonus Damage applies to the amount of Damage that will be Split. This can alter the number of targets eligible to be chosen. Example: Volibear, Furious is a unit that says in part "When I attack, deal 5 damage split among any number of enemy units here." A player attacks with Volibear, Furious while they also control Annie, Fiery, a unit that says "Your spells and abilities deal 1 Bonus Damage." Volibear, Furious now deals 6 damage split among any number of enemy units at its location, and can choose to split that damage among up to 6 units rather than the usual 5.