It's Airfields in your data base suitable for use by your Aircraft type as specified by your Airline.
Mine has any Airfield with runways 5,000' or longer suitable for the A330. ( although having said that the actual runway PCN may not be suitable for A330 use !! )
The FM Nav data base will have at the the very least the runways and hopefully all the approaches/sids/stars, even if you don't have a paper chart it's useful in an emergency.
Limited by data base size, so your Airline decides which ones to include.