Much like PD and PD_ONLY, a hint that specifies a weapon on autofire should only ever fire at fighters. As opposed to the "PD, PD_ONLY, ANTI_FTR" combination which would also fire at missiles.
And before you say this wouldn't be useful to vanilla: Swarmer SRMs autofire at non-fighters which wastes the cooldown and ammo. In my opinion they ought to use this hint so they're reserved for firing at fighters.
This hint could possibly intelligently detect if an enemy fleet has no fighters, and thusly OK its usage against non-fighters.