Fortnite custom Resolution to 100% correct?

I do not know exactly how I should go, and although I have the "problem" not to know what the box shown in the picture is, some Pro players have in their custom resolution there's not a hook in turn others not Would be glad if someone know more about the selection box, maybe I could explain it and give an advice whether it is meaningful or not, maybe it has no effect on the custom resolution, or no influence on the game. (1440/1080)

I also play on 1440x1080 and I can say IT WORTH IT. You have to set full screen as you are playing x1080. Therefore, the full screen.

Yes I mean did you tick the box below? What I marked in red because some do not do that

Yes, do that!