Ok, so by "borders" you mean something else than the lines displayed on the map, showing where one realm ends and another begins?
I'm talking about provinces and how they're divided. The Duchy of Normandy is set to be a possession of the Kingdom of France. Regardless of who actually controls it, France still has rights over it. They have the ability to take actions to reclaim it, and they have the ability to pass laws which will affect the duchy. As mentioned previously, the developers have said that kingdom laws affect every duchy in the kingdom regardless of who actually owns the duchy.
from dev diary 3
"Only the holder of a Kingdom title is allowed to change these laws, and they will affect the whole geographical kingdom, regardless of whether a province is actually under its de facto control"