"Start from scratch” means you cannot make a game before the announcement. Some submissions may have already existed, but the question is: how can the organizers know if a game was truly made from scratch for the fairness of the jam? “Start from scratch during the event” means you cannot develop the game, especially the core game logic, before the jam officially begins. I know this because I’ve participated in some jams before, and many people have cheated by preparing games ahead of time, sometimes even using projects made a year ago, just remodeling or adding random elements, and still managed to win.