3−2−2. Class A Airspace
a. Definition. Generally, that airspace from 18,000 feet MSL up to and including FL 600, including the airspace overlying the waters within 12 nautical miles off the coast of the 48 contiguous States and Alaska; and designated international airspace beyond 12  nautical miles off the coast of the 48 contiguous States and Alaska within areas of domestic radio navigational signal or ATC radar coverage, and within which domestic procedures are applied.

Class A airspace is the most tightly controlled type of controlled airspace, and is restricted to only IFR traffic.  Operations in Class A airspace require both the pilot and airplane to be IFR certified and operating on an active IFR flight plan.
All aircraft operating within Class A airspace are required to set their altimeter to 29.92.  This standard has been adopted to allow for easier separation from Air Traffic Control and reduce the altitude deviations that can come from an incorrect altimeter setting.
There is no need for a clearance to enter Class A airspace, clearance is implied when you file a flight plan with a cruising altitude over 18,000ft or ask for an altitude over 18,000 when flying IFR.
Among the different classes of controlled airspace, Class A airspace is probably the easiest for a pilot to fly in.  Class A airspace was designed to offer a safe environment for high speed aircraft operating IFR at higher altitudes.  The speed and altitudes associated with Class A airspace make see and avoid nearly impossible.  At 30,000ft the sky looks a lot bigger and it’s much more difficult to pick out individual aircraft.
The only time that a VFR aircraft is allowed to operate in Class A airspace is during an emergency in which the pilot must climb in to Class A airspace for terrain, communications, or other emergency related situations.