Насколько я понимаю, сейчас принята точка зрения, что общественные отношения в парфянском и раннесасанидском обществе сильно напоминали феодальные. А так как в античном обществе ничего подобного не было, Плутарх просто употребил наиболее близкий по значению термин.
Вот что, кстати, написано в "Кэмбриджской истории Ирана" (т. 3(1), стр. 528): Virtually all the attributes of medieval European feudalism can be found in Parthia, Armenia and Iberia. Allodium and fiefs, investiture and homage, immunity and vassalage, all these familiar concepts have their Parthian and Caucasian counterparts. Feudalism in its most flourishing age was, of course, anything but systematic, and it is an institution very difficult to define. However, certain fundamental principles have been distinguished by medieval historians, and these apply quite well to both Parthia and to Armenia and Georgia. These include: the relation of vassal and lord; the principle that every holder of land is a tenant and not an owner, until the highest rank is reached -sometimes the concept even rules in that rank also; that the tenure by which a thing or estate of value is held is one of honourable service, not primarily economic, but moral and political in character; the principle of mutual obligations of loyalty, protection and service binding together all the ranks of this society from the highest to the lowest; and the principle of contract between lord and tenant, as determining all rights, controlling their modification, and forming the foundation of law.
