$o - opponent's primary weapon (if vnum is zero, they are unarmed)

$p - secondary weapon/shield (if vnum is zero, then neither is used)

Also, in the HIT trigger if/when it's added:

$o - the weapon that HIT them (be it melee or ranged)

$p - shield of opponent (vnum == 0 if they have no shield or the weapon is ranged)

if vnum $o is zero, then it was hand-to-hand damage.

The reason for all this is that things can be done based on the weapon(s) used. Even the shield can be manipulated. For instance, if the mob does a block/counter and bashes the shield.