Millitarized zone or Province. A good example would be the province of Brabant in the Dutch Republic after the 80-years war for independence. Every Province in the Republic was pretty autonomous but Brabant was under direct control of the state. They also had less representative rights. This was for defensive purposes but also because it was largely catholic.
I think that's what they mean with the march building. Which does make me feel like it should lower autonomy by - 10 a month in said province to represent the direct control by the state in order to establish it as a frontier.