On a Machine Running Several X Servers, How Do Programs

On a machine running several X servers, how do programs identify the different instances of the X11 server?

A) By a fixed UUID that is defined in the X11 configuration file.
B) By a display name like : 1 . By a display name like : 1 .
C) By the name of the user that runs the X server like x11:bob. By the name of the user that runs the X server like x11:bob.
D) By a device name like /dev/X11/xservers/1 . By a device name like /dev/X11/xservers/1
E) By a unique IPv6 address from the fe80::/64 subnet. By a unique IPv6 address from the fe80::/64 subnet.