The "Strike Fighters 2..." series is a series of flight simulators developed by Third Wire Productions, Inc.
Originally games were released under the Strike Fighters / Wings Over series. But in time, development of the games meant migrating some stuff. The Strike Fighters / Wings Over series became a separate 'pre-Vista' market line, the Strike Fighters 2 series Windows Vista and higher. With this chasm a number of changes appeared:
SF2 series is purely single player,
the SF2 games can be merged with other games from the series
for the SF2 series additional expansions and DLC is made and sold