On Star Wars MUSH, Features are those characters established in the Star Wars universe — such as Luke Skywalker or Mara Jade — by official Lucasfilm sources (movies, novels, RPG sourcebooks) that are considered canon. Since their presence tends to have a dramatic effect on both their factions and on roleplay MUSH-wide, Feature Characters (or, FC's) are restricted to players who have obtained the permission of the MUSH administration to play them.
Because of their effect on the overall balance of the MUSH, the number of Feature Characters is limited, particularly those with Force abilities. Applications for FCs not currently advertised for are acceptable but must include information on how the new Feature would enhance RP for the faction to which s/he belongs and for the MUSH overall. Any FC alive during the MUSH's timeline may be considered for play, though the number of FCs is limited to keep them special and prevent them from overwhelming the non-FCs' roleplay.
See this List of Feature Characters for those that are currently within the MUSH storyline.