Three identifiers travel with every commercial vessel: the IMO number, the MMSI, and the international radio call sign. They look similar in passing but they identify different things and they have different lifetimes.
IMO number
Seven digits with a check digit. Identifies the hull. Issued at construction. Never changes. Never reused. The right key for permanent-record research.
MMSI
Nine digits. Identifies the radio installation. Issued by the flag state’s telecommunications regulator. Changes when the vessel is reflagged. The right key for AIS reception logs and VHF radio licensing records.
Call sign
Short alphanumeric string. Issued by the flag state’s telecommunications regulator alongside the MMSI. Used in voice radio communications. Changes when the vessel is reflagged.
Putting them together
When you cite a vessel anywhere it could be re-read at a later date — a paper, a research note, an internal database — always anchor on the IMO number first. Add the MMSI for AIS-related research and the call sign only for radio operational context. PortWatch shows all three on every vessel profile to make cross-referencing straightforward.