What is single/doubled pixels setting?

There's "single pixels"/"doubled pixels" option in "display setup" menu (the background is in-game footage; note how UI changes as well):

"Single pixels" looks sharper, but what is actually changed? It may appear that the game lowers rendering resolution when "doubled pixels" is selected.

Solution 1:

It means that in low detail mode(DOUBLED PIXELS), you'll receive four times less of rendered pixels, while keeping same screen resolution. This, so called "term", actually arises from old times of 320x200 VGA displays, when decreasing virtual resolutions of screen to render, caused the game to have much higher FPS.