What Is the File Extension BUP?by Mike Smith
The file extension-the last three letters of a file's name-identify the file to a computer or other device, so the computer can choose the correct program to open the file. A BUP, or .bup, is a backup file.
BUP files are a backup of the information contained on a DVD video. If a computer or DVD player cannot read the primary information on the disc it will attempt to recover the missing data from the BUP file.
BUP files are closely associated with the IFO files they back up. IFO files are responsible for telling the DVD player which screen to display at startup and direct the player to the main content of the disc.
In order for a disc to be DVD-compliant, or playable by standard DVD players, it must contain BUP files.