Whereas Kanmotsu (tribute goods paid as taxes or tithes) and Nengu (land tax) are considered as the transformation of the tax, So (rice tax) of the Ritsuryo system (a system of centralized government based on the ritsuryo code), Zoyaku indicated Zokuji (all miscellaneous dues in the form of non-rice products and labor), Buyaku (labor service), Rinji Zoyaku (extraordinary irregular levies) that are considered as a continuance of a per-capita tax, Zoyo (irregular corvee) (regarding what the taxes of Yo [labor or alternative goods] and Cho [textile goods or alternative money] transformed into, there are two theories: one theory asserts Kanmotsu and another Zoyaku).
律令制の租税である租が変質したと考えられる官物・年貢に対して、雑徭など人身別賦課を引く雑公事・夫役・臨時雑役などを指して称した（庸・調の後身に関しては官物説と雑役説の両方がある）。