Within each faction in 40k (or just about anywhere, really), there's inevitably going to be sub-factions. In the T'au empire, they're called Septs and each one gets its own rules to make them unique. Get a preview of these Septs in this look at their upcoming codex.
From the article:
The T’au Empire is a beacon of innovation in the otherwise luddite 41st Millennium, constantly developing new and tremendously powerful weapons, evolving its tactics, and bringing new worlds under its sway.
Though united by the Greater Good, each T’au sept world has a distinct cultural identity which influences all aspects of life – including its approach to war. The Empire’s many septs fight in markedly different ways, and the upcoming Codex: T’au Empire gives you a toolbox of options for how your army approaches the business of battle.
There are six major septs in codex T’au, from the ubiquitous T’au and Vior’la to the more specialised Dal’yth and Bork’an. Each gains a sept tenet, a Warlord Trait, a Signature System Relic, and an exclusive Stratagem, representing the character of the worlds they live on.