The BUP file extension is used for one of three types of files on a DVD video disc. It is used for files that back-up the IFO information files on the disc. A DVD video disc could therefore theoretically exist without BUP files, although it would be less reliable.
Not all DVDs will have files with the BUP file extension. This is because DVD stands for Digital Versatile Disc and can be used for multiple purposes, including pure data storage. The BUP file extension is part of the specification for DVD Video, which is the format used for discs intended to be played on a standalone DVD player.
A DVD video file will contain three types of files. The most important is VOB, short for video object. These are the files that contain the audio and video content itself. The video from a disc that has only VOB files can still be played on most DVD players and computers, but with limited functionality.
The second type of file is IFO, which is abbreviated from information. It contains details about the video content. These details include the times at which any chapters start and begin. They also include the information needed to distinguish between different audio tracks, for example where there are language or commentary options, and different subtitles. The IFO file will also denote which video file is to serve as the on-screen menu.
The final type is BUP, short for back-up. This is simply a back-up of the IFO file, meaning each DVD video disc has one IFO file and one BUP file. The BUP file exists solely to make sure the disc can still work fully if the IFO file is corrupted in any way. This could happen if the disc surface is smudged or scratched.
Another important difference between the file types is the issue of encryption. VOB files are encrypted to prevent the movie content being copied or "ripped." IFO and BUP files are not encrypted.
It is also possible for the BUP file extension to appear in other software contexts. Some applications will create back-up files of key data and will simply add the BUP file extension after the main file type extension. This avoids any confusion that could cause the computer to access the wrong file. As and when the back-up is needed, the BUP file extension can be removed from the filename, meaning the back-up file simply takes over from the original.