Only land holding characters, and only direct vassals of the clan leader. So handing out daimyo titles to a few vassals and then letting them have their own vassals lets you expand the clan's territory while keeping the number of direct vassals down (i.e. the number of people you really need to keep happy. these are also the only ones that can plot against you). In the clan view you can mouse over the clan heir and pretenders to see exactly who is voting for them.