Final answer:
High-level airspace in Canada starts at 18,000 feet above sea level, managed by NAV CANADA and is typically used by commercial flights.
Step-by-step explanation:
In Canada, all high-level airspace starts at an altitude of 18,000 feet above sea level, which is known as Flight Level 180. This airspace is managed by NAV CANADA, which is responsible for the country's air traffic control services. High-level airspace is designed for the regulation of air traffic for aircraft flying at these altitudes, generally used by commercial and long-distance flights.