It is in Configuration > Order: Venues and decides whether a venue reservation and catering food can be combined in one online order:

• Not Allowed — a venue can be reserved on its own; if the cart contains catering meals, selecting a venue fails with Catering Is Not Allowed On Venues, and the meal picker is hidden once a venue is selected.
• Mandatory — the venue pop-up is only offered when the order contains catering meals; a venue cannot be booked without food.
• Optional (default) — after choosing a venue the customer is asked Do you want to Add Catering ? with Yes / No.