The territorial zone or the territorial waters are the waters stretching from an established base line to a determined distance in which a country can define its own laws and can administrate justice. According to the United Nations Convention on the law of the Sea, the maximum size of a territoral zone is 12 nautical miles (approximately 22 kilometers), measured from the low tide mark. This is also the case in Belgium.
Note: this does not give Belgium the right to deny vessels access to its territorial waters since the right of innocent passage is still applicable. Vessels can sail through Belgium's territorial waters, but are to respect certain stipulations defined by the coastal state, such as the prohibition on oil spillage or the prohibition on fishing without a permit.
Read more on the working area of the coast guard.