When it is Yes (the default), the customer's order detail page shows Reorder buttons for that module: one for the whole module section and, for Custom Meal Builder, By The Pounds, Catering and Realtime, one per line so a single pack/meal can be re-added. Clicking it opens a Reorder pop-up that copies the previous selection into a new order. For A la Carte the same setting also adds a Reorder button (opening Reorder from previous checkouts) on the A la Carte ordering page. Set it to No to hide those buttons for the module.