Hit-markers show when you get the last shot.

Basically what it says on the tin. Say, for example, you were fighting a skeleton lord, and you and your crew are shooting it, the player that got the last hit would possibly have a red hit-marker, to show that they killed it. This wouldn't be too hard to implement, since you already get an audio cue when you kill a player, so it would be a nice touch to see it added to other things in the game.