Psychologists have in the past come up with many different lines of development. Many of them the Integral theorist Ken Wilber integrated in his so-called AQAL Matrix by finding similarities between. He postulated that all, or at least many, in some way move along the same stages.
For a long time, I struggled with clearly categorizing the lines of development. I knew about Wilber's categorization in the four quadrants (individual-subjective, collective-subjective, individual-objective, collective-objective), but that didn't seem satisfying to me, especially because of just categorizing roughly.
After in the meantime learning a lot about the Enneagram, I began to notice different lines in myself that seemed associated with the Enneagram types. Today I believe that there is exactly one line for each of the Enneagram types 1 to 8. I couldn't find any line for type 9, and my theory is similar to that of the cognitive function stack here: Type 9 is no opinionated identity on its own like the other 8 types, but rather a coordinator between all those identities. Finding peace is not a motivation that competes with any other motivation, which is why it's not a distinct ability, so a line for that doesn't really make sense.
1: Moral development
2: Empathy development
3: Development of image self-esteem
4: Development of identification
5: Cognitive development
6: Development of the inner debate
7: Development of taste
8: Self-control development
More lines are possible, but not yet defined by me.