Priority

Prority is a property of hitboxes in the Super Smash Bros. games and in Super Smash Flash 2 which describes the behavior when they interact with each other. The priority of hitboxes produced by normal ground attacks follow a set of rules that are dependent upon the amount of damage they deal (the law of high and low priority). However, different rules apply to those of normal aerial attacks and certain special moves when they are performed in the air. Some hitboxes and attacks, such as shots from Fox's Blaster, possess transcendent priority, which ignore the rules of normal priority.

Priority applies to individual hitboxes within attacks, not the attack as a whole. However, in the event that a single hitbox is "out-prioritized", it is possible for the whole attack to be cancelled.

There is no true priority in the original Super Smash Flash, attacks simply pass through themselves, in the case of projectiles, and character model, they do not cancel each other. In all cases, the deal out of attacks will be determinated by which input was triggered first.

Normal priority
Normal priority describes the set of rules that apply to normal attack hitboxes which means that any attack hitbox that is not classified as transcendent. The majority of standard attacks, aerials, special attacks and projectiles contain hitboxes with normal priority.

Ground attacks and projectiles
The hitboxes of normal ground attacks follow the law of high and low priority. Which means that they interact with each other in terms of the damage they deal. When two ground attack hitboxes collide they will either cancel each other out, or one will override (out-prioritize) the other. This collision is signified by a white "bubble" and a distinct "ting" sound.

If one attack hitbox is above the priority range of another then the stronger hitbox out-prioritizes the weaker one and the weaker attack is cancelled by the stronger one. However, if two colliding ground attack hitboxes are within the priority range, they will "clash", and both will cancel out with characters not receiving any damage at all.

For example:
 * If 's Rasenshuriken collides with 's fully charged Getsuga Tenshō (Projectiles 30% vs 30%) they should cancel each other out. However, the Getsuga Tenshō out-prioritizes the Rasenshuriken.
 * If Naruto's uncharged forward smash clashes with Ichigo's uncharged forward smash collide (14% vs 14%) both attacks will cancel each other out.

Aerial attacks
Different rules apply to the hitboxes of normal aerial attacks. When a normal aerial attack hitbox overlaps that normal ground attack or another normal aerial, the attacks cannot collide or clash with each other and the law of high and low priority does not apply. However, if this ever occurs, both will persist, irrespective of each, and will damage opponents if the move connects. The hitboxes of aerial attacks can collide with normal projectiles so in this case the law of high and low priority functions. However, aerial attack animations cannot be canceled out, and will continue even if out-prioritized.

Transcendent Priority
Transcendent priority describes the set of rules that apply to transcendent attack hitboxes, as distinct from normal attack hitboxes. Transcendent hitboxes ignore the rules of normal priority: they cannot collide with, clash with, cancel out, or be canceled out by other hitboxes.

list of character's attacks that contain hitboxes with transcendent priority.
 * - Bomb
 * - Blaster
 * - Hammer (both grounded and aerial), Final Cutter (both the sword meteor smash and the projectile)
 * - Spin Attack (initial hitbox only)
 * - Cape, Star Spin, forward smash (sweet spot only)
 * - Dimensional Cape, Shuttle Loop, all standard attacks except dash attack and glide attack, Up and down throw.
 * - PK Flash, dash attack
 * - up smash (sourspot only), up tilt
 * - Thunder, neutral attack, down smash, forward smash (sweet spot only)
 * - Needle Storm, Vanish, Chain
 * - neutral air
 * - Wario Waft
 * - Nayru's Love, Farore's Wind, neutral attack, forward tilt, up tilt, dash attack, forward smash, up smash (all but the final hitbox), neutral air, up air
 * Some Final Smashes

Trivia

 * For some reason an Augmented Getsuga Tenshō can be out-priotized by a regular Getsuga Tenshō.