(info "(coreutils) who invocation") says If given no non-option arguments, ‘who’ prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display.
Say if this means remote hostname or remote X display. or X display or remote hostname. By the way, now on Debian sid: $ who $ ps a PID TTY STAT TIME COMMAND 593 tty7 Ss+ 0:10 /usr/lib/xorg/Xorg :0 -nolisten tcp vt7 1310 pts/0 Ss 0:00 bash 1338 pts/1 Ss 0:00 su - 1339 pts/1 S+ 0:00 -su 2005 tty1 Ss+ 0:00 /sbin/agetty --noclear tty1 linux 2073 pts/0 R+ 0:00 ps a
