The ICAO (International Civil Aviation Organisation) is an agency of the United Nations. ICAO was founded in 1944 and has the mission to organize the civil aviation. The ICAO organize also the SAR-Service. The ICAO-Alphabet is worldwide used in the aviation.

Alongside the IATA-Code exist the distinct four-digit ICAO-Code. This code is organized as following:

• 1. digit = Region or Continent or Country (Special treatment for USA, Russia, China, Canada)
• 2. digit = Country (Special treatment for USA, Russia, China, Canada)
• 3. + 4. digit = assigned by the country

The meaning of the first digit is:

• A = Southwest-Pacific
• B = Polarregion
• C = Canada
• D = West-Africa
• E = North-Europe
• F = Southern Africa
• G = Westafrican coast
• H = East-Africa
• I = not used
• J = not used
• K = USA
• L = South-Europe
• M = Central-America
• N = South-Pacific
• O = Middle-East
• P = Northern-Pacific
• Q = not used
• R = East-Asia
• S = South-America
• T = Caribic
• U = Russian Federation
• V = South-Asia
• W = Southeast-Asia
• X = not used
• Y = Australia
• Z = China and Mongolia

I can not see the system behind this. Some countrys in the UN get a special treatment as usual.

The meaning of the second digit is assigned to one country. Learn more at Wikipedia.