A projectile is a ranged attack, usually fired from a fighter like a shot. Projectiles are generally used to threaten opponents with long-range damage.
Projectiles vary wildly in properties, such as damage, velocity, acceleration, trajectory, and on-screen duration. For instance, some projectiles can stick to enemies, some can track and pursue foes, and some can detonate into an area-of-effect explosion that damages both friend and foe. Most projectiles have damage drop-off as they fly—the farther they travel, the less damage they deal.
In broad terms, slower projectiles that stay active for a long time are effective for distracting foes, providing the projectile user an opportunity to rush in with an attack. In contrast, shots that launch and fly quickly are effective for dealing damage to enemies since they’re more difficult to react to, but they don’t create follow-up openings. Tossed items, which are in effect projectiles, also work this way. A slower toss can be followed in, allowing you to pressure the foe’s attempt to shield or dodge the thrown item. Meanwhile, a smash throw deals increased damage and is faster to hit, giving them less time to defend, and you less time to capitalize on the brief on-screen threat.
Projectile hitboxes are treated as a different type of attack in terms of collision interactions. Most projectiles collide and nullify each other, while bigger projectiles such as Samus’s fully charged Charge Shot can plow through minor shots unscathed, still decimating opponents in the way.
Most projectiles can be reflected by deflection attacks like Falco’s Reflector and Ness’s side smash. Reflected projectiles deal higher damage, according to the reflector’s properties. Reflected projectiles can be reflected right back; if a shot bounces between reflectors several times in a row, watch out! It’s almost become a mini Final Smash!