Where is the value that defines SCREEN stored?
 

In the effort to bring up XWin functionality, trial and error process reealed "export DISPLAU=192.168.1.101:0.0" to be the final setting.

Is there a variable or configuration file that defines the value that the variable "DISPLAY" must match in order to get X apps to work?

Is there a definition for each of the zeros in :0.0?
What is the difference between :0.0 and :10.0
would there ever be a case for :0.1 or :11.2?

Not sure, but I think the :0.0 are xy coordinates.
If you look in your xorg.conf, in the "Screen" section you should find somewhere:
I think it's that.
Usually when I set my display I just put:

Thanks for the response!
My xorg.conf lies in /etc/X11, but contains no "Viewport" and as you can see from below, the closest I come is the BusID "PCI:1:0:0" under "Device" --- the Sections for "Screen" and "Monitor" have no reference.
Like you I do "export DISPLAY=:0.0" and it works but there must be something to compare "DISPLAY" to in order to get a match.
I seem to have several files that contain xorg.conf in their names. What path are you using to locate your xorg.conf?

----------------------------------------------------------------------
partial xorg.conf below
------------------------------

Section "Device"
Identifier "VIA Technologies, Inc. UniChrome Pro IGP"
Driver "via"
BusID "PCI:1:0:0"
EndSection

Section "Monitor"
Identifier "DELL 1901FP"
Option "DPMS"
EndSection

Section "Screen"
Identifier "Default Screen"
Device "VIA Technologies, Inc. UniChrome Pro IGP"
Monitor "DELL 1901FP"
DefaultDepth 24
SubSection "Display"
Modes "1280x1024" "1152x864" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
EndSection

Section "ServerLayout"
Identifier "Default Layout"
Screen "Default Screen"
InputDevice "Generic Keyboard"
: