The design documentation exists but has never been completed and cleaned up, and since they affect what I do for this phase 2, I'm working on it.
As a reminder; settlements are locations that regroup infrastructures, so they can illustrate a city a base or anything like it. They are represented by an icon on the surface map of the Orbital Object Panel and it can be only one settlement by region and by faction.
There are 5 types of settlements in the game + a special one that I will not cover here since it is linked with the space units and is post alpha 10; surface settlement, space-based surface settlement, subterranean settlement, lava tube settlement and asteroid settlement.
Each of them has different levels of development, specific requirements and particularities.
Each type of settlement indicate in some sort the shape of it in the orbital object's landscape. Certain types are specific for certain environment and one is specifically targeted for the asteroids.
So during the setup of a Colonization Mission, it will be possible, after this iteration, to choose which type of settlement will be used.
So that's it, work work and work :)