In fact, the IMO number is a unique seven digit number that is assigned to propelled, sea-going merchant ships of 100 GT and above upon keel laying, with the exception of ships without mechanical means of propulsion; pleasure yachts; ships engaged on special service, such as lightships; hopper barges; hydrofoils, air cushion vehicles; floating docks and structures classified in a similar manner; ships of war, troopships as well as wooden ships.

The IMO number is a mandatory prerequisite for sailing through the SOLAS regulation XI/3 which was adopted in 1994; in fact, specific criteria for passenger ships of 100 gross tonnage and upwards and all cargo ships of 300 gross tonnage and upwards were agreed. Further to this, the records based on the IMO number also require an independent audit trail for each vessel.

The scheme was further applied to fishing vessels in 2013, and the eligibility criteria were amended in 2016 to cover smaller and non-steel hull vessels. In particular, the IMO number is assigned to the total portion of the hull enclosing the machinery space and can play the determining factor, should additional sections be added.

For new vessels, the IMO number is assigned to a hull during construction, generally upon keel laying. The SOLAS regulation XI-1/3 requires ships' identification numbers to be permanently marked in a visible place either on the ship's hull or superstructure. Passenger ships should carry the marking on a horizontal surface visible from the air and vessels should also be marked with their ID numbers internally.

This number remains unchanged and is never reassigned regardless of any changes concerning the ship’s owner, country of registration, name or transfer to other flags and should be inserted in the ship's certificates. It is also important to note that this number is separate and different from the official number issued by the vessel's flag administration which is only internally used and cannot replace the IMO number.

IMO notes that the IMO ship identification number scheme, which was introduced in 1987 through adoption of resolution A.600(15), can act as a measure aimed at enhancing "maritime safety, and pollution prevention and to facilitate the prevention of maritime fraud." It aims at assigning a permanent number to each ship for identification purposes.

IMO numbers can be an essential tool in shipping because they help to improve monitoring, control, surveillance and enforcement of operations, as they allow flag states to accurately manage vessels under their authority; give national authorities information to help them police their waters more effectively; bring clarity and consistency to legal records; and help governments determine whether vessels are authorized to be in their waters.

All in all, having an IMO number is a prerequisite for selling and can be the best way to track and locate a vessel’s history since each number is unique and is the only identification that remains with a vessel from shipyard to scrapyard, regardless of all other modifications. Indeed, the IMO number is recognized by most governments and regional fisheries organizations and is considered the best available global identification system for ships.