A FireMissile

A_FireMissile is a code pointer used in the attack animation for the Rocket Launcher.

spawns a rocket (object of type ) moving in the direction that the player is currently facing. Autoaim is applied to determine the vertical angle to aim at. The rocket is spawned in the exact same X/Y location as the player; it passes out of the player since rockets pass through the player that fired them.

Uses
The  code pointer appears in the following states in Doom's state table:

Example
The following is an example of how to set the  code pointer in a Dehacked file:

Pointer 21 (Frame 34) Codep Frame = 61

Or using BEX syntax:

[CODEPTR] Frame 1234 = FireMissile