Whether reserving a venue is required depends on "Catering: Ordering On Venues": Not Allowed (venues aren't offered), Mandatory (you must pick a venue to check out any catering order), or Optional. Booking a venue checks for time-conflicts with other events at the same venue in the same window; a conflicting, unavailable, or unselected venue returns a specific error ("This venue is unavailable for the selected date. Please select a different venue.", "No venue is selected, Select a Venue Please.", or "The given Venue is Booked at this time interval, try another one please.") and blocks the update.