Based on reports from an owner who's already received his, it seems that setting the screen resolution to "Best for display" on the new Retina 5K sets it to 2160 x 1440, exactly the same as the resolution of last year's 27" iMac. As an example and to verify what that member of the Apple forums is describing, with an attached secondary Dell 2560 x 1440 monitor at native resolution, when he drags a full-screen window from that monitor to the Retina 5K's built-in display, it takes up the entire screen. IF that 5K monitor were truly running at 5160 by 2880, that window should take up ONLY 1/4 of the 5K's screen. This would mean that each image pixel is composed of 4 physical pixels in a 2 x 2 grid. As all four would have to be displaying exactly the same color, this would give image clarity not one whit better than that of the display for which the Retina 5K is supposed to be an "upgrade." What's worse, is that the Retina 5K apparently cannot even be driven at 4K resolution, let alone 5K. Ironically, hooking up a 4K monitor as a secondary display on one of these so-called "Retina" iMacs will give an effective pixel density closer to "Retina" (however you define that) than will the allegedly advanced display on the Retina 5K. Why was Apple not forthcoming about this? How can Apple term this display "Retina???" Putting tiny pixels on a display with a density >200 is meaningless unless each pixel can be driven individually. Unless Apple has a great explanation for this problem, this 5K display is not a technological advancement at all. And how can they logically claim to potential customers that they're offering a "Retina" display when its resolution is the same as that of the last 27" iMac, about which they made no such claim. Does Apple have a legal problem here? While I await delivery of my recently-ordered 5K, I certainly hope that Apple publishes an explanation or clarification. Until then, I'll hope there's something I'm not understanding clearly. Independent reviewers have claimed that this is the best display ever. I certainly hope that their opinion isn't placebo effect based on what they've been told they're seeing.