The cast members are great about telling you what is included on the dining plan, and which items on the menu are eligible to be counted as the drink, dessert, and/or entree.
What they may not tell you about -- and what you'll want to research ahead of time -- is what the rules are for substitution. (e.g., at CS restaurants, you may substitute your drink or dessert (or both) for snack items from the same menu). At a la carte TS restaurants, you can ask to substitute your dessert for a side salad, soup or fruit, if offered -- just ask your server if any DDP dessert substitutions are available at that venue.
As others said, if you want to "get your money's worth" from the DDP, you'll need to be visiting the most expensive 1-credit TS restaurants , (exception: dinner at CRT and brunch at the California Grill are both 2 credits, and worth the DDP expenditure as they cost 2x the average TS meal out-of-pocket), at the most expensive time of day (dinner, for those that don't have an all-day menu), and ordering the most expensive items everywhere you go. If saving money on the DDP is not important to you (e.g., you wanted it for the prepayment factor and not to save money), then you don't need to worry about any of that.