Yakusa no Kabane (the eight honorary titles) (八色の姓)
The article in November, 684 in "Nihonshoki" (Chronicles of Japan) has the following description.
The Emperor made a decree, saying 'The hereditary titles of all the families are reformed and Yakusa no Kabane instituted.'
The First is Mahito. The second, Asomi/Ason. The third, Sukune. The fourth, Imiki. The fifth, Michinoshi. The sixth, Omi. The seventh, Muraji. The eighth, Inagi.
August, 682: The hereditary titles were valued to select the government official.
October, 684: The title of 'Mahito' was given to 13 clans, such as Moriyama no kimi, Michi no kimi, Takahashi no kimi, Mikuni no kimi, Tagima no kimi, Ubaraki no kimi, Tajihi no kimi, Ina no kimi, Sakata no kimi, Okinaga no kimi, Hata no kimi, Sakahito no kimi, and Yamaji no kimi. 公 is pronounced as 'kimi'.
November, 684: The title of 'Asomi/Ason' was given to 52 clans such as Omiwa no kimi.
New order of the social status
The titles actually given were the higher rank of four titles, 'Mahito,' 'Asomi/Ason,' 'Sukune,' and 'Imiki' described in the above chronology. While the old order of the social status was Omi, Muraji, Tomonomiyatsuko, and Kuninomiyatsuko, the Emperor Tenmu selected only the ones who had a close connection with imperial family from among Omi and Muraji, gave a new titles of Mahito, Asomi/Ason, and Sukune to them, created a new order of the social status, and raised the status of imperial family. The Emperor clarified a family lineage between the superior and lower-level officials and made a sharp distinction between the central nobles and local powerful clans.
However, the Emperor did not apply this system to all titles, and the existing titles remained. Therefore, the old titles such as Omi, Muraji, Tomonomiyatsuko, and Kuninomiyatsuko all remained. It can be presumed that the Emperor made a distinction from old clans by establishing new titles which were ranked higher than the existing titles, Omi and Muraji.
It appears that only the clans who had the titles of Mahito, Asomi/Ason, Sukune, and Imiki could produce the officials of nishiki no koburi (the seventh grade of twenty-six grades of cap rank) in the future Kan-i system (Office and Rank System).
Transition from the system of clans and hereditary titles to ritsuryo system (ancient East Asian system of centralized governance, in Japan: esp. 7th-10th century) and the bureaucracy
As seen in the beginning of the selection of Asuka Kiyomihara Code (the legal code of Japanese ancient state) in 680 as well as in the edict of Yakusa no Kabane, it was one of the policies to reform the old clan system and to create the bureaucracy which was able to promptly respond to a new national regime.
When four major family titles which were Minamoto clan, Taira clan, Fujiwara clan, Tachibana clan had gradually prospered from the Nara period to Heian period, most of the titles of their descendants became Asomi/Ason, and the title itself became meaningless.