Population

Goal of the population system is to represent the role of people who live and work in each provinces, as well as how they can vary and what kind of effects that can have. Population is divided into classes, where each have wealth, income, spending, and consumption, while growing or declining based on those various factors. They can migrate to other provinces, or get promoted/demoted based on provincial factors.

Classes
Class represents a distinct socio-economic group that is different from others in a certain way. There are in total 9 classes, divided into 3 strata. Each strata have their distinct role, as well as each individual classes within.

First, there's the rural strata. Its role is to partake in primary industry to produce rural goods such as food, fuel, fiber, salt, etc. They consume those rural goods themselves, and sell the surplus to other strata which they consume for various purposes. They are the most numerous of the three strata, while also being the most basic yet foundational strata. They are the backbone of any society, above which all else is built upon.

Then, there's the urban strata. Its role is to take the surplus output to produce more sophisticated type of goods such as consumer product, naval product, industrial product, etc. They also function as a market to suck up the surplus good produced by the rural strata. They generate a lot of wealth, but they also consume a good portion of that vast wealth for their own usage. They are hallmarks of a sophisticated and developed society, but a brittle one too.

Lastly, there's the notable strata. They are notable people of a society, an upper elite, who hold wealth and power. They are far richer than any other classes in other strata, taking a portion of their surplus wealth. They are also vital to maintaining a sophisticated society, allowing long range commerce to occur while producing vast amount of knowledge. They also, due to their vast wealth, serve as a primary consumer of luxury goods.

Growth
Each class has birth and death rates. This is further impacted by...

Gravity
Employment rate, life fulfillment, and wealth impact gravity.

Migration
People move to locations related to gravity.

Migration pool: There are several sources for migration flux. These are your provinces and your subject's provinces (vassals, feudatories, tributaries and colonies), the HRE (if your provinces are in the HRE, and they are not in Italy). Subcontinent migration is not implemented atm.