Damage is the basic measure of how vulnerable a character is to the knockback of attacks. At low damages, the character cannot be knocked back very far, and can only be thrown off-stage by the game's most powerful attacks. However, at high damages, attacks deliver considerably more knockback, increasing the risks of severely damaged characters to be KO'd.
Damage is represented by a numerical percentage that starts at 0% and can rise all the way up to 999%. Despite this formatting, getting to 100% does not mean a character is doomed — the number itself is somewhat arbitrary. This value is displayed as an integer in every game prior to Ultimate, though is only actually measured as such in the original game. In Melee, Brawl, and Smash 4, it is actually a decimal value that is merely rounded down when displayed. In Ultimate, the value is displayed to one decimal place.
Each attack deals a set amount of damage, which is then modified by things such as Stale-Move Negation before the target is launched. An attack that does no damage will not cause targets to flinch and not produce a regular hit sound, although it will still cause knockback. The damage an attack deals is a significant factor in how much knockback it causes.
The following aspects are affected by damage: