ADS Contract or ADS-C is part of the FANS-1/A equipment and is referred to as
ADS-C reporting is controlled by the ground station in all situations other than emergency contracts. Only the flight crew can declare and cancel ADS emergency reporting. Although the crew can initiate the emergency reporting mode, the aircraft cannot initiate a contract. If there are no current contracts with a ground station, the pilot can "arm" the mode and then the mode will affect any subsequent contracts.
There are three basic contract types; the Periodic Contract, the Event Contract and the Demand Contract. There are also a number of "on-request" information groups within the FANS-1/A ADS functionality that can be added to the Basic ADS Report for downlinking as part of an ADS report.
The event contracts available in the FANS-1/A system are:
• the Vertical Rate Change Event,
• the Lateral Deviation Event,
• the Altitude Range Change Event, and
• the Waypoint Change event.
http://www.acac.org.ma/SEMINAIRES/NS...%20photos).pdf