Minecraft wiki entry for Player.dat_format, associated text:  Fireworks: One of these may appear on a firework rocket.
 Explosions: List of compounds representing each explosion this firework causes.
: A firework explosion.
Tags common to all firework explosion effects[show]
 Flight: Indicates the flight duration of the firework (equals the amount of gunpowder used in crafting the rocket). Can be anything from -128 to 127.