I think it's probably more "chosen by your clan" kind of thing- they raise you as a candidate to become a cleric, wizard, and the Queen says, "I'll grant 3 initiates this year to your clan" and they pick the folks who are best fit. As far as historians, anyone who takes the time to apprentice can become one.

But that's a good question!

Whenever I first put words down about a setting, there's a lot in my head and I often have a hard time remembering what I've written vs. what's still unwritten.