IMO: The IMO number is a seven-digit unique reference number. It is assigned to each ship for identification purposes and is permanently associated with the hull.
MMSI: MMSI is a unique nine-digit number for identifying a ship. It is programmed into all AIS systems and VHF electronics on board of the vessel. The number is international standardized for contacting vessels. In some cases, it is possible that the MMSI number of a vessel changes, e.g. the vessel is sold or long-term chartered and the flag changes.
ENI: The ENI number is an eight-digit unique registration number for inland vessels. The number is permanently assigned to a vessel for its lifetime.
Vessel ID: The vessel ID is an internal number used by the FleetMon database to identify vessels. Every vessel that receives an entry in the database gets a Vessel ID assigned to it. The assignment of the Vessel ID is based on the MMSI. AIS signals received from vessels already stored in the database are assigned to that existing Vessel ID.