The DEP and ARR buttons are specific to the TOWER role. They will be visible or not depending on your profile, and enabled or disabled depending on the status of a given Flight.

The DEP button is used to notify the Time of Departure and Date of Departure of the flight: 

The ARR  button is used to notify the Time of Arrival and Date of Arrival of the flight. The Aerodrome of Arrival can be changed in case the flight has been diverted:

The Additional Addresses link opens a dialog allowing you to specify the additional AFTN address(es) where the Departure or Arrival notification will be sent when the OK button is clicked.