As a member of the Fire Caste, it is your sacred duty to protect the other castes dedicated to the other three elements and one otherworldly one. You have gone through more than two thousand years of Tau history and are a member of an elite fighting unit created specifically to be the most capable and powerful of all Tau. The entire Tau Empire depends on your skills, strength, courage and devotion.
And why should you care about protecting other castes? Maybe because all individuals from this society have their place and importance in it. For example, members of the Earth caste are usually craftsmen and workers. Their domain is the construction of machines and cities, which is why they are very necessary.

Equally necessary are the members of the Water caste, who form the bureaucratic, merchant, and political stratum of the Tau. As you might guess, the Air caste is in charge of transportation, and finally, there is the caste of the wisest of all the Tau, the ruling caste, who lead the entire civilization. You have to pay special attention to these.