A caste system is a social system. It only says this person is social above or below you. It doesn't say how those people make money. Communism is a political/economic ideology. It definitely orders how money is made/distributed. It also has a type of caste system in party members/nonmemebers. But thats a superficial similarity and not the defining characteristic. Communism may have a caste system but a caste system isn't necessarily communism.