The Portion Sizes admin page has two tabs: Meal Prep and Catering (the Catering tab only appears if the Catering module is active). A catering portion size is a name (e.g. "Half Tray," "Full Tray") plus an optional Default Container Type. One catering portion size is automatically designated the account's default catering portion — the size the system treats as the "regular" catering size for a meal (for example, when laying out the pricing table or checking swap/modifier eligibility). In the current version there's no on-screen control to change which portion size holds that designation; it's set automatically (typically to the first catering portion size you create), and the list page marks it with a "(Default Portion)" hint.