There are 3 possible events you can create. Here they are and explained how:

• Mission Planner: The mission planner is for Main Missions. You have to follow this guide line for the requirements. These events also need approval by CMs.
• Training Planner: The Training Planner is for Training events. If you are running a Training use this calendar.
• Community Events: This is for games other than Arma 3 or special events in Arma 3 which take place during off hours. This includes reserving the server for you and your friends to play together.

If you do something on the Training Planner or the Community Events Calendar, it is preferred that you either take the 2nd Server or make sure nothing else is running at the same time.