Final answer:
When in flight within the altimeter setting region, the altimeter should be set to the local altimeter setting provided by air traffic control or the airport.
Step-by-step explanation:
When in flight within the altimeter setting region, the altimeter should be set to the local altimeter setting provided by air traffic control or the airport. This ensures that the altimeter accurately displays the altitude relative to the local pressure, which can vary due to changes in weather conditions. If the altimeter is not set correctly, the displayed altitude may be incorrect, potentially leading to errors in flight navigation.