At first, it was the second highest, below Mahito, which was given to members of the Imperial family, but after Heian period it became the highest of the eight noble titles:[1] Ason, Mahito, Sukune, Imiki, Michinoshi, Omi, Muraji, and Inagi.
[2] The title was created as a part of the eight-kabane system, proclaimed in 684 during Emperor Tenmu's reign, as its second highest rank.
As such, Asomi was initially given to the highest ranked noble clans whose genealogical origins were often claimed back to imperial princes.
[2] Although the clans closest to the Imperial House, that is, descendants of Emperor Keitai, were intended to be promoted to Mahito, the first rank in the original eight kabane system, this fell out of favor with the nobility.
Since the Heian period, Ason became the highest of the eight kabane, and began to be used by imperial princes and their descendants.