ISO w/ .MDF file

[3lutz3toe]

I’m currently downloading a DVD that comes w/ an ISO file that is 6.8GB in size. It also comes with an .MDS file. I have a question as to what the .MDS file is for and do I need it to burn the ISO file in Toast 7?
I have googled on this and the answers vary. One say that you don't really need it. Another say that the .MDS file contains info about dual layer (DVDs ripped w/ dual layer).
Any insights?
Thanks.
I’m planning to either burn it to a dual layer dvd or compress it into a single dvd5. Not sure yet. I think I might want to preserve the best quality and burn it to a dl dvd.

[on30trainman]

When I do a build in ImgBurn I get both the .ISO and .MDS file. I have only used the build mode recently when making an ISO that needs to be burned to a DL disk so it may be the info on where the break is. If I try to open the ISO in ImgBurn to do the burning it tells me to use the MDS file.