Throughout Three Decks all naval vessels are arbitrarily classified using the British rating system in force at the time of their launch or acquisition (or by the 1626 system if prior to this date) which includes 'Unrated' as the smallest classification. Vessels that were not formally a part of a national navy are classified by their normal purpose. These separate classifications are; 'Privateer', 'Hired vessel', 'Merchantman', 'Letter of Marque' and 'Other Vessel'; the later also including some purpose built naval vessels which can not be considered as a warship (Sheer hulks for example)
In addition ships may also be classified according to their own national rating system where these exist or hired vessels according to their nominal rate while in service (although this is yet to be implemented in full).

All searches and other references to a ships rate refer to the arbitrarily assigned British rate.