It is to belong to Aaron and his sons as their due share forever from Bnei-Yisrael, because it is a contribution. It is to be a contribution from Bnei-Yisrael from their sacrifices of fellowship offerings, their contribution to Adonai.
Thus you will also present an offering to Adonai from all your tithes that you receive from Bnei-Yisrael, and from that you are to give Adonai’s portion to Aaron the kohen.