After you have consumed a meal and either added each food eaten to the Meal Builder or just need to mark a planned meal as consumed:

1. Click on the meal to bring up the Quick View
2. Check the "Consumed?" box in the lower-right corner of the Quick View. There is also a "Consumed?" box in the detailed view. 

This converts a meal that is "planned" to one that is "consumed." Any changes you make to the meal before it is "consumed" will be reflected in the planned meal. After "logging" the meal as consumed, you may edit the meal and the changes will only be reflected in the "consumed" meal.