Under the Ritsuryo system (a system of centralized government based on the ritsuryo code), onshu was the registration of a person who came forward after having been missing from the family registers or the keicho (yearly tax registers), or after having been away from the place of registry. The Japanese reading of onshu is 'kakuretaru ga araware' (literally, "They were hiding, but they appeared").
In contrast, the registration of individuals who were uncovered by a Guji (the chief of those serving at a shrine who controlled festivals and general affairs) was called kasshutsu, and reinstating individuals who had become one-time vagrants as residents of their original family register was called sokan. There was no substantial difference among these three types of people, and population increases in the family registers associated with their registration reflected on the ability rating of the Kokushi (provincial governors) and Gunji (local magistrates) (kokaryo (regulation on the efficiency rating of government officials)).
In the Yoro ritsuryo code (a code promulgated during the Yoro period), there was a regulation on onshu, but the oldest documented record dates back to 726 when the Taiho code was in effect. The purpose of the regulation was originally to secure tax payers, but during the early Heian period, onshu and kasshutsu were used as a legal loophole to migrate to the Kinai region (provinces surrounding Kyoto and Nara) where the tax burden was lighter than in other provinces. A number of people even had their names entered in another family's registry to illegally obtain kubunden (farm land given under the Ritsuryo system) or onni (a system of passing court ranks onto a new generation through ancestors under the Ritsuryo system).
Consequently, onshu and kasshutsu were temporarily banned in 800 and only sokan was permitted. Onshu and kasshutsu were reinstated in 806, but were banned again in 855.