I have always assumed that would let me toggle between my skin (the one I used last for that aircraft I think) and the one the mission builder selected (the one in the .mis file). That being said I have never figured out how/if it works.
Skins should be selected in the arming screen where you can visually see what you actually select if you ask me but let's leave it the way it is for the moment. Thanks vonofterdingen.
And what the "RANDOM" button does?
In QMB, press the "Random" button and it will randomize the number of aircraft per flight, skills, planes, loadouts and countries (now called Air Force). If you click Random in the main page, it will only randomize the first 8 Flights. If you click Random in the second ("Next" button) page, it will randomize all Flights in 16-Flights missions. Things can get a little crazy, the Random button should be named
Mayhem!