If the X client application doesn't specify any font to use, the MacX
server looks for the "fixed" font. See page 115 of "Apple MacX User's
Guide" for details.
Note that even if the X client doesn't specify the font name on the
client's command line, the client can still look for some default places in
the client host (like .Xdefaults file in user's home directory and the
/usr/lib/X11/app-defaults file) for resources. These files store
application and machine-specific resources. The details about how the X
resources are referenced can be found in most X11 reference documents, such
as the one on page 10 of X(1) manual pages in "X11 Reference for A/UX."