No, you pick which show you want to attend, and you'll be given a time frame during which can book your meal. For the 5:00 show, I believe your ADR time would be between 1:45 and 2:45 (and I mean that you can book anytime in that time frame, not that you would have to book for 1:45 and finish by 2:45), for the 6:45 show the time would be between 3:30 and 4:30 (which would be considered dinner), and for the 8:15 show the time would be between 5:00 and 6:00.
ETA: I just confirmed these time frames at easywdw.com (this post is from last year, but I would assume that it will be the same for this year.):
http://www.easywdw.com/category/worth-2/candlelight/