class IMyProject(Interface):
manager = ContactChoice( title=_(u"Project Manager"), source=ContactSourceBinder(portal_type=("held_position",),
relations={'position': '/contacts/ecreall'}),)
Means that 'manager' is a multivalued contact field wich vocabulary get held_position of site restricted to ones wich have a 'position' relation to '/contacts/ecreall' (i.e. wich are held_positions in ecreall company)