Here are a couple of articles:
The basic difference as I understand it is a sump just adds volume to your setup and a fuge is lighted and has LR
etc to grow things that might not flourish in the main tank in an environment sperate from your main tank but still adds volume to the setup.
Since you are using the same container I guess the only difference would be if you leave it bare it will be a sump. If you put substrate, LR
etc and proper lighting it will be a fuge.
Someone correct me if I am wrong.