These are the most commonly used nomenclatures, apart from ‘umu’ (family unit), in Igboland. Both words ìsú and ìdú appear to be cognates of one another. For instance, both words to some extent are said to mean ‘forest’ and/or ‘assembly’, with ìsú being the most widely used but the must ambiguous in meaning.
The Igbo are a forest people, or forest dwellers so it would make sense for the early Igbo society to use forest units as demarcations. The Yoruba are an African group known to practice the same tradition by using the nomenclature ‘Igbo-‘(Forest) as a demarcation of a unit area: Igbomina, Igboho, Igbokun, etc.
In Igboland we have ìsú and ìdú: Ìsúikwuato, Ìsú Njaba, Ìsúochi, Ìsú Awa, Ìsú Onicha; while on the other hand we have Ìdú Osobile, Ìdú Obosi-Uku, Ìdú-umuje (Idumuje), Ìdú-umuobi (Idumu Obi), etc. This all indicating a unit or assembly of people. Ìsú Ikwuato: Forest unit of three lineages or Ìdú Obosi-Uku: Forest unit of the Great Obosi. Call it forest unit, forest gathering, or forest assembly all the same.
Some may ask how /s/ turned to /d/ and vice versa but remember that’s the dynamics of language. For instance, when it comes to the word follow, some places say ‘Dòó’ instead the commonly used ‘Sòó’. There are many examples that I’m sure others can give.