On Blood of Dragons, Feature characters are defined as characters that are mentioned (not necessarily by name) by GRRM, either in the books or elsewhere, such as in correspondence with fans. For the time-period of the MUSH, they include all Targaryens as well as some other characters such as Alyn Velaryon and Cregan Stark.
As part of our agreement with GRRM and to maintain the canon history (some of which is known only to the Admin), we use Feature characters primarily as background characters. No player—and this includes Staff—plays a Feature character as a regular PC. Instead, Staff emits or puppets these characters when they are needed to advance plots and/or to add general atmosphere to the game. At times, experienced players may also be asked to help Staff run these characters in a similar fashion.
On the whole, we feel that this is the best solution since most of these Feature characters hold positions—King, Hand, Kingsguard, etc—that are poorly suited to characters being played on a day-to-day basis. However, their presence is very much an integral part of the setting and as such we try to integrate them into the game as much as is reasonable.
To learn who the Feature characters on the game are, you can use the +cdb/search command to list them by typing +cdb/search type/feature.