Ed......No you dont have to start from scratch if you make a mistake on a track as we all do when recording.......its sought of a suedo Multi track recording operation that works very well.....

You record your music first.....then SAVE/NAME it as a song WAV file.....
Then you would Record your vocal track using the saved music track as your backing track.....then SAVE both.....
then Record the harmony track using the new saved music/vocal track........

Remember you can also record using all modes at once too!!! WAV/MP3/SMF/SFX/DJ loops/Styles mixin & matching as you go along....its really awesome if you have a good creative imagination!