The marks are solely for the Tether / Charge combo, which only appears after second adds. You will ALWAYS see heat lightning / tether after second adds, then a heat lightning / critical rip. Then the randomness begins.
Usually you get a heat lightning / charge, or a heat lightning / tether, but sometimes there is no heat lightning and you will get one person with a purple ball over his head for charge at the same time someone else gets a blue line for tether.
At this point you need a priority system to figure out who stacks, and when. The tether will always go off first. Let's say someone who is not marked gets the tether, and someone who is not marked gets the charge. 1 and 2 will stack in the center, just behind and to the side of the person with tether (so they get knocked backwards instead of knocked into the boss' purple persistent aoe). Then after this happens, the boss will shoot the little lightning stun bolt at the person stacked on C for charge. 3 and 4 will run in at this point and stack to mitigate the charge.
That is ideal. But never ever happens that way. Often, one of the numbered people get the charge. #3 has the toughest job. If 1 or 2 are tagged with a blue line tether or a purple ball indicating charge, then #3 needs to stack with tether. If 1 and 2 do not get tagged, they will both be stacking for tether, so #3 will stack for charge.
If you are #1 or #2, unless you are targeted with the line or the ball, you will always stack for tether. You will never stack for charge.
If you are #4, you will only stack for tether in the rare event where #2 and #3 both get targeted with a tether and a charge, or vice versa. Otherwise you are always stacking for charge.
So the brunt of the thinking is placed on #3, and #4 has it tough as well, but a bit easier. #1 and #2 only ever have to remember to stack for tether unless they are targeted, so they have it easy, and the people who are not marked just have to worry about where to stand if they have the purple circle or the blue line.
That's the best I can describe what is going on there without drawing a picture.
Edited, Feb 20th 2015 9:19am by Valkayree