I don't know if all of you guys are getting it. lol
It's either the way the menu selection looks/works in-game, or it's just a visual representation used at E3 to show the different features.
What LilKi's saying is, what feature do you think the last un-named island represents?
If I were to guess, it could be an alternate version of the map editor made specifically for Singleplayer/Co-op mission creation. They have stated that there's no bots in the multiplayer map editor, but this isn't the traditional multiplayer map editor, it would be a storymode editor. I think they would make it entirely separate because of how much more advanced it would have to be with AI path nodes, scripted events, ect. TimeSplitters did it like 10 years ago, I see no reason why FarCry 3 can't today. The varying terrain can easily be navigated by AI if we give them path nodes.