From OpenStreetMap Wiki
|Indicates the IATA airport code|
|Used on these elements|
|Tools for this tag|
Theis a three character code used to uniquely identify most major airports across the globe. It is often based on the name of the airport or the name of the city.
The key iata=* has as it's value the IATA code of the aerodrome being represented by the object. These tags could be added to the node or area of the airport (the object with the tag aeroway=aerodrome) .