Boundaries and Flows
People and groups construct territory by creating, maintaining, and defending boundaries. Boundaries are made visible (or demarcated) by things such as fences, flags, posts in the ground, and gates.
In geopolitics, boundaries are dividing lines between territorial entities such as places or states. People and groups construct territory by creating, maintaining, and defending boundaries.  For example, states require boundaries to provide legitimacy for their control over their citizens. In other words, a boundary recognizes the territorial extent of sovereignty. This includes which people are under the control of the state and which resources they can use.
Boundaries are made visible (or demarcated) by things such as fences, flags, posts in the ground, and gates. Sometimes demarcations are frequently tied to the physical geography of the landscape. Physical barriers such as mountains, rivers, and lakes are frequently used, but each presents their own issues. For example, if a river is a boundary, what is the location of the boundary when the course of the river changes because of natural processes of erosion?
States secure their boundaries based on their relationship with their neighbors. Peaceful boundaries are formed and cross boundary interaction can be positive when there are no disputes over where a boundary lies. To be peaceful, boundaries must support a sense of security for individuals, and cross-boundary trade and movement are seen to be positive. States can support cooperation by sharing resources across the boundary, creating laws which allow for the free flow of goods, and coordinating local administrative efforts such as transportation. For example, a landlocked state can negotiate with a bordering state to gain access to a sea port across its border. Boundary conflicts can occur when states the geography of national or ethnic groups does not match the geography of states. Such as national or ethnic ties to certain land, disputes over resources, and superimposed borders which were placed with little attention to cultural heritage.
Oceans have their own form of territory (what we call territoriality), which makes a distinction between national and international waters. States have an area around their coastline which is designated as an exclusive economic zone, or EEZ. They can claim rights to resources such as oil or fish found within 200 nautical miles of their land-based territory. The geopolitics of EEZs can be seen in the geopolitical codes of China and its building of small islands in the South China Sea. If these islands are recognized as sovereign territory, China’s EEZ would extend to 200 nautical miles past the limits of the island. This would change the geography of EEZs and access to maritime resources (fish and fuel reserves under the seabed) that are currently based on the coastlines of countries in the region.
 Colin Flint, Introduction to Geopolitics, 3rd edition, pp. 139-173