The 'common mark' is the letter which indicates nationality and the aircraft's registration follows it.

For an aircraft registered in the UK they might be G-ABCD where G is the common mark and ABCD is the registration.

The ITU (International Telecommunications Union) allocates common marks to countries and may allocate more than one option to a country.

The country then decides which one it will use but they have to notify the ITU and ICAO which one they intend to use.

The country then allocates the aircraft registration letters or numbers.