I think there is some confusion on what rank a boss mob really is with the current descriptions. Pretty much they have bars that look about the same other than the add little boxes on the end. Part of the description talks about the length of the bar. There is no difference in the bar length, or at least not much. It's the little boxes accenting the ends of the bar that matter - 1, 2 or 3 boxes.
So the trend I have seen now that we have a lot more events like my Map Romp on Tuesdays, is that everyone is claiming these bosses differently.
Dolmens and World Bosses are easy to claim, they have their own icon. 3 Dolmens and 6 world bosses per zone. Some people are not properly marking world bosses with the world boss icon.
Now the first confusing part is there is the icon for delves, but we seem to all be trending to mark we did a delve plus add a boss. Now you don't get credit for the delve without killing the boss, so I'm not sure if we claim both or not. Anyway, if we also claim the boss it better be the wimpy green head because these guys can be soloed since they are pretty much only decorated with one little box on the health bar.
Public dungeon bosses need a group. Most are two boxes, but I think they sneak in some with only one box decorating the bar. So we have to watch these. I have claimed this with the orange head boss due to most have two boxes decorating the bar.
Only thing I have ever claimed as a the red head is the group challenge boss who has three boxes on his bar.
Seeing that we have people claiming even delve bosses as orange instead of wimpy green and then others not even claiming above green for even the tougher bosses. This really needs some better clarification.
Again to sum up what I think it should be, and I could be wrong, is the following:
Health bar has one little box on either side - Green
Health bar has two little boxes on either side - Orange
Health bar has three little boxes on either side - Red
I have not seen any other major difference in the health bar other than these boxes unless they are pulling a fast one in trials, and I haven't got there yet. =)