Drew, it really varies from show to show. There is no universal set definition of classes. It all depends on the organizers of that particular show. Generally box stock means nothing but what came in there kit box with the exception of coatings like paint and bare metal foil and sometimes flocking. But no aftermarket stuff like wiring, photo etch etc or parts from other kits. Usually the instructions have to be displayed with the model.
After that show organizers are usually the ones that determine what they expect in a class. I've found that at most shows the people running the show will do their best to help you place the model into the correct category if you ask at the registration table.