Pies are defined by their crusts. A filled pie (also single-crust or bottom-crust), has pastry lining the baking dish, and the filling is placed on top of the pastry but left open. A top-crust pie has the filling in the bottom of the dish and is covered with a pastry or other covering before baking. A two-crust pie has the filling completely enclosed in the pastry shell.

Regular pizza is bottom-crust, calzone is two-crust. That's my interpretation.