A PosAttack

A_PosAttack is a code pointer used in the attack animation for the Trooper.

causes the monster to fire a hitscan attack against its current target, internally calling  to face in the direction of the target. It is similar to the A_FirePistol code pointer used in the pistol firing animation sequence. The sound effect DSPISTOL is played.

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 = 185

Or using BEX syntax:

[CODEPTR] Frame 1234 = PosAttack