For example, in a family of 4, you place a VMS and set serving size to 4. Every meal- breakfast lunch, dinner, dessert- made on that lot will have 4 servings. Let's say you have 3 guests come at dinnertime. Even though you made 4 bowls of cereal and 4 grilled cheese sandwiches, you can set the VMS to 7 to make 7 hamburgers for dinner. All meals on that lot will have 7 servings until you change it again.
Previously made meals (if unserved) retain the size they had at creation.