Primary is to Secondary as Principal is to...what? [closed]
Primary is to Secondary as Principal is to subordinate/supplemental or subsidiary. ..a level down.
Primary Division and Secondary Division vs. Principal Division and supplemental Division.
It's more a matter of choice and prevalent usage.
If you mean 'of lesser importance' I think Subsidiary works as an opposite of principal.Auxiliary is another such word. But in general these mean lower in function or importance and not lower down in a graph hierarchy or a map. In that sense principle and secondary are better as you indicated.