BUP (Back Up) is the filename extension of some files stored on video DVDs.
The BUP file contains the information about the organization of tracks, menus, chapters, subtitles on the disc. It is created automatically by various applications and generated in case the program crashes or the original file becomes corrupt and needs to be restored.
BUP files are used in the event that the corresponding IFO file is unreadable, perhaps due to a scratch on the surface of the disc. Like IFO files, BUP files are not encrypted.