A_ChangeFlag
From ZDoom Wiki
A_ChangeFlag (string flagname, bool value)
Changes the specified flag and sets it to the given value. Not all flags will produce useful results when changed in the game.
Example
actor CowardImp : DoomImp
{
states
{
Pain:
TROO H 2 A_ChangeFlag("FRIGHTENED", 1)
TROO H 2 A_Pain
goto See
}
}
This imp will activate the FRIGHTENED flag when he enters his pain state, meaning when he gets hit he will attempt to flee from the attacker.

