It's left pretty vague and generalized within the game world for a reason: players shouldn't view this as hard and strict rules, they should be unsure of the whim of the gods and jotuns. There aren't any 10 commandments here, just a rule of thumb, and you can stay safe by never looting, or you can take your chances and loot and the Norn will decide if there are consequences. Remember the gods of mythology aren't algorithms with predictable outcomes, they are moody and capricious.
That said, my rule of thumb at the table is: if the slain believes in the Norse mythos, then their possessions are sacred. If they follow the Celtic gods or the White God, then they are fair game to be looted. I hope that helps and clears/muddies things up.
