Priority

Priority is a mechanic in Super Smash Flash 2 that determines the behavior of attack hitboxes when interacting with each other. A hitbox may have one of two different types of priority: normal priority, which is dependent on the damage difference with an opposing hitbox, and transcendent priority, which is a special property applied to certain hitboxes. The effect of priority varies based on the kinds of attacks being used.

The effects of priority apply to individual hitboxes within attacks, and an attack with multiple hitboxes can have different priority depending on which hitbox lands. However, the results are applied to the attacker as a whole. Additionally, priority does not apply when the opposing hitboxes avoid each other and land on their respective opponents' hurtboxes instead, which instead leads to trading, in which both characters are attacked normally.

There is no true priority in the original Super Smash Flash, as hitboxes cannot collide and simply pass through each other.

Normal priority
Normal priority describes the set of rules that apply to the majority of attacks containing hitboxes. This type of priority is based on the damage dealt by each hitbox, in which a hitbox that deals at least 8% more damage than an opposing hitbox has higher priority, whereas hitboxes with smaller differences in damage share the same priority.

How normal priority behaves depends on the types of attacks colliding. With ground attacks and certain grounded special moves, two opposing hitboxes with no priority difference will clank when colliding with each other, canceling each attack and causing both characters to rebound backward. A clank is signified by a white bubble that appears where the hitboxes collided and a distinct "ting" sound that plays. However, if one attack's hitbox out-prioritizes the other, then the attack with higher priority will follow through, and only the character with the lower priority will rebound.

With aerial attacks and certain aerial special moves, their hitboxes cannot clank when overlapping with those of a physical ground attack or another aerial attack. When this occurs, both attacks instead continue as normal or trade when connecting with the opponents' hurtboxes.

Different rules apply for projectiles. If two projectiles collide, then the one with lower priority will disappear, or they will both clank and disappear if they have the no priority difference. If a projectile collides with a physical ground attack or aerial attack with higher priority or no priority difference, then the projectile will disappear and the physical attack will continue like normal. If the projectile has higher priority, then no interaction will occur.

Transcendent priority
Transcendent priority is a property of certain hitboxes which prevents them from clanking with other hitboxes. This is denoted within SSF2's code as hitboxes having a priority value of -1.

Hitboxes with transcendent priority cannot cancel out or be canceled out by other hitboxes, instead passing through them without any interaction. This makes disjointed attacks more prone to trading and ineffective against projectiles. Likewise, transcendent projectiles will go through attacks, including other projectiles, without stopping or clanking.

Below is a list of attacks that contain hitboxes with transcendent priority.

Trivia

 * Initially, normal priority was based on a "priority" variable in the code ranging from 0 to 7, and attacks with higher values would have higher priority. This system was retired with the introduction of damage-based priority, though the variable is still used for transcendent priority.