The class A airspace is high-level airspace, above a flight level of 18,000 feet. I should point out that the different classifications of controlled airspace are all based on the types of traffic that operate in those regimes, and it's all based on the amount of control service that's provided. Some of these airspace classifications have equipage requirements, so that the appropriate control service is provided.
Class B airspace is above 12,500 feet, and in that area all aircraft are controlled. Between 12,500 feet and the surface you have class E airspace, which is airspace that does not have the same traffic levels as the class D and class C airspace that appear around aerodromes.
It's a complicated issue to describe, because the rules that are associated with each class, as I mentioned before, depend on the amount of traffic that appears there and the rules that are applied to provide that service required to ensure flight safety. Class F is a special airspace that allows Transport Canada to restrict operations, so that nobody appears in that and nobody goes in there unless authorized by the user of that restricted airspace.