Ingubunkokusei was a system of granting the right to recommend a provincial governor to a specific province to retired emperors, nyoin (women bestowed with the title "in" (usually the empress, imperial princesses, etc.)), second consorts to the emperor, and imperial princesses serving at the Ise Shrine at the time of their appointment, and in return they would receive the financial profits of the provinces in place of a salary. Officials of the retired emperor's office were usually appointed to zuryo then travelled down to the province and took control of it. This system existed from the Heian period to the period of the Northern and Southern Courts (Japan).
The theoris on the origin of the ingubunkokusei system describes: One that it started when kanmotsu (tribute goods paid as taxes or tithes) and choyo (taxes in tribute and labor under the ritsuryo system) collected within Awaji Province were used to feed Imperial Prince Oi who had been deported to the province and called Awaji Haitei (deposed Emperor of Awaji); another theory argues that the system began when denso (a rice field tax) and rice collected within Yamato Province was used to feed Retired Emperor Heizei who had confined himself in Heijo-Kyo; a third theory states that the system was established along with shinno nintoku (provinces whose governing posts were reserved as sinecures for imperial princes) system that was introduced later on. However, it is now most commonly believed that the oldest example of the system being used occurred when Shinano Province was given to Retired Emperor Uda in 908, followed by Musashi Province ten years later. Subsequently, Higashi Sanjoin FUJIWARA no Senshi was the first nyoin to be treated in the same way as the Retired Emperor. Furthermore, Emperor Toba's wife FUJIWARA no Tokushi (the biological mother of Emperor Konoe, later becoming FUJIWARA no Tokushi) was given Echizen Province in 1146. It was due to this event that empresses and imperial princesses serving at Ise Shrine were able to receive the same kind of treatment.
It is noteworthy, however, that there are two theories about what was granted from provinces. One theory argues that a portion of the salary of an appointed zuryo was granted, while another theory states that a part of the income from such sources as taxes collected by zuryo was granted. According the to first theory, income to the recipients would have been too small and zuryo would not benefit from the system, so the second theory is more plausible, however the details are unknown.
In any case, at the end of the Heian period, province-based income tended to be delayed, leading to increased cases of zuryo accepting the control of multiple provinces.
Although the term of a zuryo was usually four years, recipients took turns appointing zuryo to different provinces every year (if a zuryo controlled four provinces, the recipients could recommend and appoint a zuryo in a different province every year) causing zuryo to be called 'nenbun zuryo.'