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:
 * Stability (max bonus on +2 [+3 gives the same bonus])
 * Fulfillment of needs
 * Population Capacity (in case of Urban Population) - provided by Amenities Infrastructure
 * Province Trauma
 * War exhaustion

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

Migration
People move to locations according to gravity and inside a particular migration pool.

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

Special Migration areas:
 * HRE (only between provinces of Germanic/Low Frankish/West Slavic culture)
 * Italians (only between provinces of Latin culture)
 * Mayans
 * Irish
 * Manchurians
 * Nomads in steppe regions

Cities
Cities are places where residents and bughers live and work. Provinces with cities are eligible to gain provincial modifiers in few categories, increasing effectiveness of some of the urban industries, or providing the owner of this province with other benefits.

City modifiers
There are 4 categories in which provinces with cities can get modifiers, each appearing in a few ranks:
 * Largest City, based on urban population:
 * Provides the owner with prestige, increases local institution spread, and reduces material cost for projects
 * Largest Regional City
 * Largest City on the Sub-contintent
 * Largest City on the Continent
 * Centre of Trade, based on trade power(commerce industry):
 * Provides the owner with additional merchants. This effect is doubled for capital province.
 * Minor
 * Important
 * Major
 * Dominant
 * Centre of Production, based on urban production(urban industries):
 * Increases urban output, and reduces cost of industrial infrastructure
 * Minor
 * Important
 * Major
 * Dominant
 * Centre of Learning, based on knowledge production(Religious and Higher Education industries):
 * Provides owner with prestige, increases local institution spread and knowledge output, and reduces cost of academic infrastructure cost, but also increases local unrest.
 * Minor
 * Important
 * Major

City growth
There are two main sources of city growth:
 * Natural growth
 * As a base, it is a percentage of current urban population, modified by many factors, the one that is easiest to influence being Population Capacity
 * Population capacity of a city is determined by the level of amenities infrastructure in a province and it is a soft cap, meaning it is possible to have higher population than capacity, but doing so will increase the death rate of population, hampering city's further growth.
 * Migration (Begginer's Guide: Migration)
 * Immigration happens when a province has a high immigration pull, usually caused by high wages. To promote immigration to a city, one can invest in urban industries, to lower their throughput which in turn will increase wages offered by these industries.
 * Every province has a limit on how much immigration can it receive every year, and it is a base of around 100 + a certain percentage of current urban population. This means that at some point, investing even more into industries, will not result in higher immigration.
 * Keep in mind, that even if a city apparently meets all the requirements for requiring high immigration, you still need people willing to migrate to it. If you're trying to build up many cities at once, some of them may receive only a fraction of migrants they would otherwise. To increase the overall availability of peasants willing to move to cities, it is recommended to expand food producing industries (crops, pastures, fisheries) to increase the surplus of food and thus the amount of unemployed peasants.
 * This works the other way around as well, so if there are no jobs in a city, people may emigrate from it to find fortune elsewhere, sometimes even if it means going back to being a peasant.

Establishing new cities
To establish a new city, one must make sure there is at least one urban industry in a province (cottage industry doesn't count). If it's not, you need to open one.(Industry guide: Opening new Industries)

Keep in mind that building a city is very long term process, and it will take many years before natural growth picks up and even becomes significant when compared to immigration.

In the first few years, almost entire city growth will come from immigration, and you should be able to keep it at at least 100 from the start - if not, there may not be enough peasants willing to move. Since even with rank 0 amenities, province has 3k urban population capacity, you don't need to worry about building amenities from the start. Even if you can afford amenities rank 1, you still need residents that will provide labour to build it. Personally, I recommend building lvl 1 and 2 amenities when you reach 2k and 8k urban population respectively. When your city reaches 1,5k urban population, you should also open commerce industry, to support trade in the area to make sure that your cities can get the goods they require.

You may have realised now, how long will it take to reach even that point - with growth of 100 per year from immigration, it would take 15 years to reach just that 1,5k population required for opening commerce industry. Of course, once you get first hundred residents, both natural growth and immigration will slightly increase, and will continue doing so, but you should not expect to reach 1,5 k urban population in less than 10 years.

To keep your city attractive to migrants, I recommend investing into urban industries (Industry Guide: Investing in Industries) in such a way that they maintain throughput of around 70-80%. If it's too low, it will lower profitability of industries without increasing immigration (because of cap), and if it's too high, it won't offer wages high enough to attract all the migrants it could.

Of course there are many variables in that, and it's very likely that the "sweet spot" for you will be different, but you can treat 70-80% throughput as a starting point.

Once your city starts nearing population capacity provided by rank 2 of amenities (30k), I'd say now is a good time to overinvest into amenities - if you can afford it. These projects take a long time to build, lvl 2 -> lvl 3 requires building more units than lvl 0 -> lvl 2, so it might be a good idea to start these ahead of time.