You see the @modeltag will be substituted looking first in the mos transistor attributes - (but there is no definition there), then in the containing symbol attributes (and there is + (but there is no definition there), then in the containing symbol template attributes (and there is a modeltag=18 definition).
Now suppose you want to place another instance of inv4.sym but with a different modeltag: Since we know that spice does not allow model names to be passed as parameters we need to specialize the - inv4.sym subcircuit to a new inv_1.sym subcircuit. + inv4.sch subcircuit to a new inv_1.sch subcircuit. Therefore we give the attribute schematic=inv4_1.sch to the second inv4 instance. We also set there a different modeltag: modeltag=13
