Final answer:
The altimeter setting region refers to airspace below 18,000 feet ASL where pilots set their altimeters based on local sea-level air pressure to ensure common altitude readings for safety.
Step-by-step explanation:
The altimeter setting region is an airspace of defined dimensions below 18,000 feet Above Sea Level (ASL). In the United States, for example, pilots flying below this altitude are required to set their altimeters according to the current local air pressure at sea level, which is provided by Air Traffic Control or local weather observation stations. This procedure ensures that all aircraft in the region report altitude readings that are relative to a common standard, which is crucial for maintaining safe vertical separation between aircraft, especially in crowded airspace closer to the ground.