The first rank is basicly the planet the tau player grew up in.
The second rank is the call of the greater good, What made the Tau guy leave his world.
The third rank is the action the Tau guy embarked on, his greatest achievement.
The fourth is the aline species he encountered, be it in war, trade or just being stuck with them for a time.
The fourth is caste.
The kroot path is a bit different, only one planet. Second rank asks the kroot player if he started as a Tau kroot or a merc,
The third rank asks him if he believes in the greater good, or in profit.
The fourth rank defines how he got the attention of the Tau empire, the last his carreer.