includedefault inserts the contents of the default CON file as if it were part of the file currently being compiled. It is only useful for command-line overrides, otherwise its use will produce an infinite recursion error.
For Duke Nukem 3D, EDUKE.CON is first tried, and if it is not present, GAME.CON is used. For NAM, it is NAM.CON. For WWII GI, it is WW2GI.CON.