Do 'man X' and look for the section DISPLAY NAMES. The format is
like this: hostname:displaynumber.screennumber
Hostname is, well, hostname. Displaynumber refers to a particular X
server, which I guess is what you are calling an "xwindows session". If
you have more than one X server running on a host, then you would have
more than one display number. The "screen number" refers to a
particular screen belonging to a particular display. I'm sure it's used
sometimes, but I can't remember ever seeing it anything but zero.
Do you have a particular desire or need to change the $DISPLAY
setting, or were you just asking out of curiosity? If you want an
application to appear on a different display, you usually have to change
the display number, not the screen number; ie, the first zero, not the
second. Unless the display is on a different host, in which case it's
probably display 0 on that hostname.