.mds is for DVDR .iso images same what .cue is for .bin CDR images. Basically .mds contains information for DVD burning software and tells what .iso files it should burn to the DVDR disc, in which way and in which order.
Most of the time, .mds file is "useless" for .iso burning, as the basic DVD-Video .iso images tend to be very simple in their structure. But it should be noted that if the person who has created the .iso image has used FAT32 file system, it is possible that there are more than one .iso file that make up the whole DVDR disc and in this case, .mds file is extremely important as it "tells" the DVD burning program how to handle this type of situation.
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