[Denvercoder9]

> Often for large airports the code is very similar to the IATA code for passengers

This is mostly only the case in the United States, since it has a single-letter K prefix, so they can append the 3-letter IATA code to make a valid 4-letter ICAO code. In the rest of the world the second letter of the ICAO code is used to designate countries, so the 3-letter IATA code cannot be appended.