From the introduction:
Most role-playing games start with the hero you create. In Tsolyánu [one of the Five Empires of M. A. R. Barker's Tékumel], however, your individual hero is only half the story. Your character’s clan is of crucial importance to their life. It is a combination caste, social role, and vastly extended family that shapes everything about your hero. Thus, the first thing to do is decide what kind of clan your hero calls their own.
You’ll want to check with the GM first to see if he or she has decided on a particular list of clans to be involved in the campaign, or if all the PCs should come from a single clan. Such a choice can simplify things during play, albeit at the cost of the interesting tensions that can come about when players all have their own family ambitions to pursue.