ISO = image file, with all the data inside, including disc formatting information
MDS = additional info files, includes the layer break data
For a single-layer 4.38GB disc (called "SL", "4.7GB" or "DVD5" size), you just need the ISO. There is no layer break, therefore no need for MDS.
For a double-layer 7.95GB disc (called "DL", "8.5GB" or "DVD9" size), you need both the ISO and the MDS. If you don't have the MDS, then
ImgBurn will have to guess. It's usually a good guess, but maybe not the best one as found on the original DL author.
DVD Decrypter (or
ImgBurn) only makes an MDS if you tell it to do so, in the options. I leave mine enabled at all times, and just delete MDS files when I rip an SL disc.
