My own headcannon

Many countries are established when a group of people migrate to an area and grow as a people; however, unification of countries refer to the process which the countryhuman takes control of different regional countries and form together to become one. An example that would be used would be the formation of France.

(Source: Wikipedia, Credit: Gabriel Moss)

As you can see, the Kingdom of France was made up of smaller duchies and counties:
- Brittany
- Normandy
- Anjou
- etc..

So the Kingdom of France (the countryhuman) would gain the control of the people in the areas, making the countryhumans who originally had control to only be personifications of the area, no longer being the head of its people. This is done by the status ceremony stated earlier in the book. These countryhumans then become depend on the new country leading the country.
    - almost like an entity line in the way that they depend on the country, but through its people not the country itself
        - E.g. If the country misuses the people in the area/drastic population decrease, this would impact the countryhuman representing the area.

This is not restricted to when they first unify, it can still apply when they conquer and control region around them.
Also it is possible for the countryhuman controlled by the country to get the status back from them, so they become 'independent'. However, this would weaken the overall power of the country, so some may not give it back.

Now back to the example, the Kingdom of France can for a sort of council with the countryhumans, that they can advice what is best for their regions. However, they can choose to have, for example, Gascony killed and it would not impact their control over the region.

This can be implied to countries that have been formed by the unification of smaller counties, and this happens a lot after the collapse of an empire (for instance, the Kingdom of Prussia appeared after the dissolution of the Holy Roman Empire).

Most notable countries that became the head of the country through this route:
- Kingdom of France
- Kingdom of Italy
- United Kingdom