The X window system in a desktop Linux (where just one physical monitor is used) usually uses display 0, screen 0.
The output of
who in Ubuntu 14.04 is
user1 :0 2016-06-15 14:25 (:0)
:0 is the abbreviation for
:display.screen). Here I logged in only from the GUI.
Then I opened a terminal emulator; I ran
screen and I created two different windows (each of them simply contained
bash). The resulting output of
user1 :0 2016-06-15 14:25 (:0) user1 pts/1 2016-06-15 14:26 (:0) user1 pts/11 2016-06-15 16:31 (:0:S.0) user1 pts/11 2016-06-15 16:31 (:0:S.1)
Why is this syntax used? It seems to be
screen emulate another display inside the physical display?