How do you open a .BUP file?

When I decrypt my dvd's; I have noticed at the top of my files it says TS.BUP. What exactly is this? I try to open it and it never lets me. It tells me that files may be incomplete or corrupt.I know this is not true because I decrypted the dvd myself. I can open and view the TS.IFO and the TS.VOB just fine. I have searched and everywhere it says to use VLC to open it. I use VLC and still nothing happens, my screen just stays black. I have tried to use multiple programs within my computer to open this file but with no luck. I use windows7 64bit if this helps. Thank you for any feedback