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.