The Tau caste system divides all Tau into 5 groups known as castes. The castes are named after the elements of nature, and closest to the particular caste in question. One's caste is determined at birth and inherited by one's parents. Inter-breeding is forbidden by the Etherals. To choose another caste for himself seems to be impossible, and held contrary to the notions of the Greater Good. The castes are based upon the racial and political divisions that separated the Tau on their homeworld of T'au during the time of the Mont'au.
| order and name of rank | Fire - Shas | Earth - Fio | Water - Por | Air - Kor | Etheral or Celestial - Aun |
| trainee rank - SAAL1 | unknown | unknown | unknown | unknown | unknown |
| lowest rank - LA | warrior | worker | bureaucrat | messenger | prince |
| second lowest rank - UI | veteran | senior | envoy | carrier | prelate |
| middle rank - VRE | hero | overseer | magister | pilot | king |
| second highest rank - EL | noble or knight | engineer | diplomat | captain | holy(?) |
| highest rank - O | commander | planner | ambassador | admiral | highest(?) |
Shas - Fire caste
Members of the Fire caste are the warriors of the Tau Empire, and as such are most often seen on the field of battle. They are taller and stronger than the members of the other castes, although still weaker and shorter than the average human. These traits come from the Fire Caste's origin on the plains of T'au, where they were hunters and warriors. Tau from the world of Vior'la tend to have even greater muscle mass than the norm because of the stronger gravity of that planet.
rising through the ranks
Provided the warrior survives 4 years of active duty, he undergoes the first Trial by Fire, which can be anything from mere gladiatorial affairs to trial by constant combat. The higher one's rank within the Fire Caste, the more likely one is to be a Battlesuit pilot. Upon attaining the rank of "O", a member of the Fire Caste may be allowed to retire active duty and become an advisor to the Tau military's command structure, the Shas'ar'tol. Other than death this is the only way to leave the Tau military.
Fio - Earth Caste
This caste provides for the manufacturing, agricultural, industrial, and engineering needs of the Tau Empire, being the builders, farmers, scientists, engineers, etc. They are often described as plain and dour, possessing a rather stoic outlook on life in general. They were originally the first Tau to build cities and practice agriculture. The city of Fio'taun is an example of this, being the first place where the Ethereals appeared.
Kor - Air Caste
The Air Caste is charged with holding the Tau Empire together through the Kor'vattra, the Tau Empire's merchant and warfleet. Formerly functioning as messengers, these days they are the Tau Navy. The Tau of the Air Caste are the tallest and most slender of all Tau, with long, skinny limbs and hollow bones developed due to lives lived mostly in low- and zero-gravity ships and space stations. In the past, before the time of the Mont'au, the Tau that originally became the Air Caste had membranes stretching between their limbs, allowing them to glide on air currents. Tau pilots tend to be superior pilots compared to their human counterparts because they have a better depth perception and higher G-tolerance, but lack battle-experience compared to the best Imperial fighter-pilots.
Por - Water caste
The Water Caste serve the Tau Empire as traders, merchants, public servants, diplomats, and ambassadors. They interact with other races when such affairs are not handled by the Fire Caste. The Water Caste tend to be tall and slender compared to other Tau.
Aun - Ethereal/Celestial Caste
The Ethereals serve as "philosopher kings", and guide the other castes, as well as alien member-races of the Tau empire, towards the enlightenment of the Tau'va; the Greater Good.
Sources
- Codex: Tau (3rd Edition)
- 1Fire Warrior (Novel) page 120 (the canonicity of the information contained in the novel is unclear)