I basically comm whoever best allowed for themselves and everyone else to perform well through unique or above-average uses of their position. If the SAM made it clear to the tank in the last Grand Cosmos fight that they could take the nearest furniture and that they'd claim the one a Yaten's distance away, all without losing damage, atop performing well throughout, while no other player showed that level of competence and awareness... he gets comm. If everyone made equal use of their position, it becomes a matter of output-per-ilvl, more or less.