The Clans category is for clans/guilds in the general and traditional sense. Many clans play other games as well, and may use this category for announcements and recruitments related for Samurai Wars. There are no specific requirements for clans.
The Role-playing category is for role-playing activities. Vult is organizing a role-playing campaign over there, and have made some requirements for factions according to the theme of this particular campaign. There may be other campaigns in the future, e.g. historical based ones.