aka Act_Racist Flags, this allows you to set which player races, and which types of elementals or necro (U)/shaman (US) pets you want the mob to be aggressive to. So you can now have a barbarian hometown guard that's aggressive to everyone but barbarians, or mobs designed to thin elementals out of groups :P
On the line of the mob entry that has the Race and Weight/Height flag simply
add a list of races, separated by periods that you want the mob aggressive to,
for example:
PH 0 0 PE.PB.EA.EF.EE.EW
Setting a mob to ACT_AGG will override this, making the mob aggressive to everyone just like before, but you can do cool things like making the mob above also AGG_EVIL so it'd attack evil humans, but not good ones, and it would kill elves and barbs of all alignments. What this means is that we can now make quests that can effectively only be done by a select number of races...and we can also make mobs that target pets, like mentals and necro's undead. And of course, we can do things like make giants agg to just dwarves, etc etc etc =) Note: that this will not make the mobs aggressive to non player/non pet mobs of specified races.