Your cartridges
Several pages here ask you for a cartridge: the console you can play, and the benches in the playground. If you have dumped cartridges you own, this is where you keep them, so that every one of those menus offers them by name and you stop picking the same file off your disk. Every account has one, and the console keeps a game’s saves beside its cartridge here, so a name you register stays registered.
A shelf is private to the account it belongs to. Nobody else can list it, and nobody else can fetch a file from it: to anybody but you, a cartridge on your shelf does not exist. The files are kept on our server beside your account and are sent only to your own signed-in browser. They are never part of the site, never in its repository, and never cached by the pages that load them.
Looking for your shelf.
What it does not do. It takes the .nes file and nothing else: a separate program and picture file pair is not a cartridge until it has a header saying which board it sat on, and we would rather refuse one than guess. It does not judge whether the console has the board a cartridge names; the console says that itself when the cartridge is loaded, and the boards report says which it has. And it does not change a file: what comes back is what you put in, checked against its digest both ways.