Federation vessels used as private colony transports. They are about four times larger than a Danube-class.
The Erewon, which has also been spelled as Erewhon, has a very unusual registry prefix. While it's still unknown what NCC means (officially it has no meaning), the 'N' has been associated as a Federation vessel. The registry BDR-529 is believed to be the license plate number in the movie "Blues Brothers."