International Maritime Organization (IMO) number https://en.wikipedia.org/wiki/IMO_number
The International Maritime Organization (IMO) number is a generic term covering two distinct meanings: the IMO ship identification number, a type of hull number used as a unique ship identifier, and the IMO company and registered owner identification number, used to identify uniquely each company and/or registered owner managing ships of at least 100 gross tons (gt).
Maritime Mobile Service Identity (MMSI) https://en.wikipedia.org/wiki/Maritime_Mobile_Service_Identity
A Maritime Mobile Service Identity (MMSI) is effectively a maritime object's international maritime telephone number, a temporarily assigned UID, issued by that object's current flag state, (unlike an IMO, which is a global forever UID).