Shibun was the term for persons with the full social rank as bushi (warrior) class among the bushi during the Edo Period. Also called Shikaku. In general, they were officially allowed to have a family name and carry swords.
Myojitaito (being permitted to adopt a surname and wear a pair of swords)
In some domains, retainers were classified into 3 classes, 'Joshi, Kachi, Sotsu or Ashigaru,' but the top two classes (Kishi and Kachi) were considered Shibun. Kishi (Joshi) were upper class bushi who were allowed to ride horses and, for example, in the case of the Hiroshima domain, bushi who were given more than one hundred koku were considered to belong to this class. Kachi (Kashi) were lower class bushi who were not officially allowed to ride horses, and Sotsu were in principle foot soldiers, and this was the term for Ashigaru (common foot soldier). Kishi with a fief and vassals were allowed to have an audience with the Shogun, whereas Kachi were given kuramai (rice preserved in a depository by Edo Shogunate and domains) and were not allowed to have an audience with the Shogun, and in the bakufu (Japanese feudal government headed by a shogun), Hatamoto corresponded to Kishi and Gokenin corresponded to Kachi. Ashigaru and below were called 'Keihai' (a person of low rank) and were not considered Shibun, so there was a large difference between the two.
Below the Ashigaru class were servants of a bushi family called Chugen (rank below common soldier), Komono (a lower servant) who were privately hired by Shibun. Some Ashigaru were allowed Myojitaito, and depending upon their achievements, some were allowed to rise to Kachi, but Chugen and below were never allowed such privileges and there was a clear boundary between these two as well. Domain constituents (retainers) strictly indicated those that had Shibun, and even in a wide definition may have included Ashigaru, but Chugen and Komono were never considered retainers.
Some commoners who helped domain finances or were wealthy merchants were given Shibun and allowed Myojitaito. There were differences such as privileges for one generation only or for all subsequent generations, only family names or only carrying swords. Because it would have been difficult distinguish between houses if farmers and townspeople had no family name, the unofficial practice of using landmarks etc. as family names was common.
Among the 47 Akaho warriors, only Kichiemon TERASAKA was not Shibun but Ashigaru class. The custom at that time was that only people of Shibun were allowed Seppuku (suicide by disembowelment) and Zanshu (beheading) was for Ashigaru upon execution. Kuranosuke OISHI thought this was unfortunate and was said to let Kichiemon TERASAKA be the messenger and survive after the raid.