It switches on the custom-portion plan feature for subscriptions. When Yes (default): the Add new button on the Plans page opens a plan type chooser (Preset vs Customize) instead of going straight to the plan editor; a plan set to Customize shows a Portion Model field on its editor, and its price rows gain a Portions column so you can assign a portion size and count per meal category. The customer's meal slots are stamped with those portions during selection. Set it to No to hide all of that and run plain single-portion plans.