These are the allince events one can start for 20 Coins. Alliance admins, alliance Co-Admins and Eventmanagers can start one for free every 7 days.
During an alliance event, certain missions are generated for 3 hours. One can set the "spawn rate" to control the amount of missions: one every 30 seconds (total of approx. 360 missions), one every 45 seconds (total of approx. 270 missions) and one every 60 seconds (total of approx. 180 missions). Furthermore one can set a "spawn area" in which the missions will appear, which can be a rectangular or a circle, available in three different sizes.
Alliance events do have the advantage that they don't block slots in your own missions list. Also they allow to get seasonal missions throughout the year: You can start an alliance event of type "Spring weather" and it will generate a lot of spring missions. This is especially usefull if people want to get the golden achievement of a certain type of seasonal missions.
Additionally, they are a great way to increase the credits earnings of alliance members. If you start an alliance event of type "Fall weather" and you participate at every single of the missions, you can expect earnings of about 1.5 million credits. It can also be a nice challenge for alliances.