(2021-04-29, 4:01)Orion Pobursky Wrote: The is no functional difference. Typically, MPDs are files that have one or more LDR file defined within them. All LDraw compliant software should be able to load and manipulate both.
I would add that all MPD files are LDR files (since the ldr extension wasn't introduced until after MPD had become standard), but not all LDR files are MPD files (since LDR files don't have to contain embedded sub-models). As Orion said, from a practical standpoint, it doesn't really matter. There is no such thing as converting from one format to the other, since they are the same format. If you don't like to see a file with a .mpd extension, feel free to rename it to .ldr; all the LDraw tools should still work fine.