Alcohol .md0 file

What is stored in the .md0 file in alcohol
Is it just a split image file and if so how does it determine where the split happens…
Its created when backing up a dvd (yes i know alcohol is not really built for dvd movie backup so please dont tell me what to use etc)
Im just basically messing around with a few things anyway i backed up hellboy with alcohol\anydvd just for kicks to try on virtual drive (7 or 8 gigs) which worked no problem (well mdf did but i could not burn it for obvious reasons) but the files were split as such any ideas on why the files are split etc