Originally Posted by
n5296s
Even in the US, which doesn't have a shortage of code points, there are probably more airfields without an ICAO code than with. The rule seems to be that airports with weather (AWOS, ATIS, whatever) get codes. Others have codes like O61, C83, 3C7, with one letter (in some position) and two digits.
Also in the US, the IATA airport code forms part of the ICAO location indicator.