Many details remain to be clarified, and there are two theories about the zatsuyo (zoyo): one is that the zatsuyo as well as the soyocho (a tax system under the kinden system, or the equal-field system) was a basic tax; the other maintains that the labor to the government was basically provided in the form of the saka (labor allocated by rank, 差科), a major example of which was the shokueki (職役), or the transport of goods), being a different tax system from the soyocho.
詳細については不明な点が多く、租庸調と並び基本的な税目とであったとする説と基本的な労役は租庸調とは別系統である差科（職役などが代表例）であるとする説がある。