Class IX is a starship classification term used to describe a type of starship in relation to other starship types in similar fleets.
In the 24th century, the Paine-class starship was considered to be a class IX transwarp frigate by the Federation Starfleet, and the Moscow-class was considered to be a class IX transwarp scout. (FASA RPG module: Star Trek: The Next Generation Officer's Manual)