There is indeed a "draft" release (I think they call it a "deployment"), but AFAIK your choice is then to either publish it or delete it. If you review it and it looks good, you publish it and can't change it thereafter (no changes to files, no adding new files, no removing existing files). If, however, you need to change something in the deployment before publishing it, you have to delete the deployment and create + upload a new "draft" release / deployment (can be same version number, since the previous "draft" wasn't published). AFAIK these "draft" releases are visible only to the author.