how are the different instances of the X11 server identified?

On a machine running several X servers, how are the different instances of the X11 server
identified?

A.
By a fixed UUID that is defined in the X11 configuration file.

B.
By a unique IPv6 address from the fe80::/64 subnet.

C.
By the name of the user that runs the X server like x11:bob.

D.
By a device name like /dev/X11/xservers/1.

E.
By a display name like:1.

Explanation: