Area QNH gives you the average QNH in the area of concern. When departing an aerodrome and operating below transition you switch to area. Prior to descent point you would switch to local QNH of the aerodrome you are operating to. If this is unavailable or forecast as per the TAF, you would use the higher minima on your approach plates. If you are using area QNH at an aerodrome you must add an additional 50 feet to the minima.