The order is the order the creatures are played.
Wait, order in which the creatures are played? Or slot order?
E.g.
Play creatures in slot 1 and 2, creature in slot 1 gets killed, play a third creature that lands in slot 1.
Does pandemonium operate on the third creature first (slot order) or the second creature first (time order)?
Because I thought it was slot order, though I'm not perfectly sure.