When an X server runs, it picks out the particular "Screen" section in
XF86Config applicable to it. In that section it finds some "Display"
subsections. In each subsection it finds some Mode references.
The XF86Config man page tells how the choice of modes within a
subsection is made, but not the choice of subsection. If there are
multiple Display subsections, what determines which one gets used in
the first place?