A single Emacs can talk to more than one X display. Initially, Emacs
uses just one display—the one specified with the DISPLAY
environment variable or with the ‘--display’ option (see Initial Options). To connect to another display, use the command
A single X server can handle more than one screen. When you open frames on two screens belonging to one server, Emacs knows they share a single keyboard, and it treats all the commands arriving from these screens as a single stream of input.
When you open frames on different X servers, Emacs makes a separate input stream for each server. Each server also has its own selected frame. The commands you enter with a particular X server apply to that server's selected frame.