:2 all are display nembers (also display names for a single motinor on the same host). If multiple X servers are running on the host then their displays are numbered as incremental values like
:X starting from
:0 to uniqely identify each one.
DISPLAY NAMES section from
man 7 X :
From the user's perspective, every X server has a display name of the
This information is used by the application to determine how it should
connect to the server and which screen it should use by default (on
displays with multiple monitors):
The hostname specifies the name of the machine to which the display
isphysically connected. If the hostname is not given, the
most efficient way of communicating to a server on the same machine
will be used.
The phrase "display" is usually used to refer to collection of
monitors that share a common keyboard and pointer (mouse, tablet,
etc.). Most workstations tend to only have one keyboard, and
therefore, only one display. Larger, multi-user systems, however,
frequently have several displays so that more than one person can be
doing graphics work at once. To avoid confusion, each display on a
machine is assigned a display number (beginning at 0) when the X
server for that display is started. The display number must always be
given in a display name.
Some displays share a single keyboard and pointer among two or
more monitors. Since each monitor has its own set of windows, each
screen is assigned a screen number (beginning at 0) when the X server
for that display is started. If the screen number is not given, screen
0 will be used.