Box track is a U shape and the rollers roll inside of the track.
It’s like pocket door sliding door hardware or closet door sliding door hardware.
Flat track sliding door hardware is where the roller sits on top of the track.
The track is literally just a quarter inch thick piece of flat bar that the roller rolls back and forth on top of. It’a made to be seen and exposed.
If you don’t care what it looks like, or if you’re going to build a facia or a valance over it, you would go with box track because it’s less expensive and it does the same thing.
You can have multiple parallel tracks rolling next to each other without the rollers interrupting each other.
If you want to see it and you’re going for more of a rustic or industrial type of look, you want people to see the sliding door hardware, you want people to see the sliding door hardware, you would go with the flat track sliding door hardware.
Flat track rides on top of a bar and there’s a flange that comes down that will run into the spacers that hold the track to the wall.