Video format. DVD's are multiple files that get put together in sequence and decoded by firmware. Typically this is done at the OS level on a PC with USB dvd player or locally in a home based dvd player. The videos you can play from a usb stick are already encoded to the correct format and put together. Which is why you can't just download a single file from a BR/DVD to make a "backup". You need to use software to rip all of the separate components and stitch them together to make a single playable file. Typically MP4, AVI, or MKV.