According to the Denryo, raw grain collected as So was taken to the Shoso (public repository) and considered shozei (rice tax) but part of them was refined into white rice and transported to the capital during the designated period (February to August) depending upon the province they belonged.
田令によれば、租として納められた稲穀は一旦正倉として納められて正税とされたが、そのうちの一部を精米して所属する国に対して指定された時期（2-8月）に都へと運んだ。