simpler explanation, save an mb and an ma file. Try and open each one in a text editor. One will work Basically an ascii version is exactly the same scene (Although it may be bigger in size) but you can read it and make changes to it. We use them here so we can script stuff to happen and parse the files and make changes without opening the scenes up.
Alan
Technical Director - Framestore
Currently working on: Your Highness
IMDB