A subfaction (aka sub-faction) is a smaller faction within a greater faction. For example, Valiance Expedition, Explorers' League, The Silver Covenant, and The Frostborn are subfactions of the Alliance Vanguard faction. The Warsong Offensive, The Hand of Vengeance, The Sunreavers, and The Taunka are subfactions of the Horde Expedition faction. In their website, Blizzard usually calls this a "sub-faction".