Each project may have multiple revisions.  There are several reasons why the system allows and generates revisions.  One reason is that you may want to publish a track while still adding, or making changes, to it.  This will allow individuals to listen and collaborate with your original track while you still have a draft version that's being changed or updated.

Another revision is created when someone collaborates with your published track and submits it to you.  You may receive several collaborations on a single track.  Each collaboration will create another revision.  You then can merge these collaborations into a new revision.  See article Merge Collaborations.