All operators must have a sufficient set of en route alternate airports
Must be certificated under FAR Part 139, or meet equivalent criteria if outside FAA jurisdiction
Must be suitable to safely operate the aircraft, i.e., runway of sufficient length, width and strength (ref FAR Part 121)
Must have minimum of ARFF Index A (FAA) or Category 4 (ICAO) response capability within 30 minutes
Must have field reporting conditions (NOTAMS), Hourly weather reporting (METARS) and an instrument approach other than GPS
Airport must be available but does not need to be continuously open (plan in place to reopen key airport services in case of aircraft emergency)
Each airline makes its own airport assessment based on their operational needs, aircraft type, flight planning requirements, and other factors.