The seen list represents the movies you have seen. It can be added to this list by marking it as seen or with a trakt check in (need to be connected to trakt).
The movies you want to watch, go in the watchlist. You may not have seen it yet or want top watch it again, it is not a problem, put it in your watchlist.
Once a movie is marked as seen (via the action or via a check in), the movie is moved from the watchlist to the seen list automatically.