alt Hacker NewsMaybe they mean X11-screens. That are more or less independent screens, you can't move a window from one to the other for example.Emacs supports that somewhat with the "open new frame on display server" menu option, but usually, multiple screens are not very useful.
Nowadays, multiple monitors present one big virtual framebuffer and only one logical X11 screen.
It is a niche feature, but it is still used eg when an application wants to have good control over a screen's frame buffer to display sth in an external monitor and minimise disturbances. I think it is fairly standard in psychophysics experiments, at least with some software. That's where I worked with such a ("zaphodsheads") setup.