Each watershed is assigned a unique number, called a Pfafsetter Code, based on its location within the overall drainage system.
The seas are delimited using the so-called definitions made by the International Hydrographic Organization in 1953.
Subsequently every watershed along this coast is assigned a number using the Pfafstetter Coding System.
The smaller systems can subsequently be numbered recursively or kept together for grouping purpose.
Landmasses (Continent and Islands) are also numbered in a logical manner, along a clock-wise oriented sea.