I've been asking similar questions too so I understand the feeling!
Tribe governments were mentioned briefly in
Tinto Talks #2 as a government type of Settled Countries, the little that we currently know is that they use Tribal Cohesion it contributes to a country's name. The same Tinto Talk also mentions how government types "...determines a fair bit of what type of mechanics you get access to. As an example, a Republic does not have access to royal marriages, and a Steppe Horde has a different view on how war, peace and conquest works compared to other types of countries." It's safe to say that we'll need much more information on what those mechanics are.
Societies of Pops are different from Settled Countries in that they don't own land, can't build most buildings, most don't have laws or taxation, and they can't build cities (as said in
Tinto Talks #26). Instead, they're based upon their pops which can migrate and can force the allegiance of pops to change to them through warfare. If they achieve certain advances, they can become Settled Countries as some playthroughs have shown. As some have said, they're like Settled Countries but worse (at least in some respects)! Like with Tribe goverments, we haven't seen their gameplay yet so that's direly needed. Given the vagueness in some qualifications, some of us in the threads are arguing about some SoPs being better represented as Settled Countries.
I forget where exactly, but I thought there was a Tinto Talk that gave at least some details about Tribesmen pops.
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