Departures indicate how many own player planes can be send to that airport.
Hi, that means if u have fleet then u can connect with one of the players n send them your fleet. Your plane eta will show up in that departure slot.
To have fleets you need to unlock airplanes with silver plane earned in the game in achievements. After unlock the airplane you want you need to buy it with wollars. Once you get done, the palne will land and you can send it to a players you’re already have a connection. After you send the plane to your connection you will see the eta in departure like that
In your screenshot you have 3 available slots on 5 (3/5) to send 3 airplanes you need to buy 3 airplanes, to send 5 you will need to open the 2 additional slots left. Unlock an airplane depends on how many silver plane you have