This IMO-number is the international ship identication number that now is being enforced by the flag states according to an amendment of SOLAS.
Even commercially operated yachts of 300GT and upwards will have to comply. The IMO-number has to be displayed 20cm high on the vessel's sides or stern as well as internally.
The IMO-number has to be visibly displayed no later than the first scheduled dry-docking after 1 July 2004.
You can read more here:
Modified by Andy Lindy at 1:38 PM 11/5/2005