S-121 is an open access method of providing digital representation of Maritime Limits and Boundaries (MLBs). Maritime limits and boundaries are constructs used to define maritime zones for nations around the world. With the United Nations Convention of the Law of the Sea (UNCLOS), they can form an international legal foundation of the marine domain. S-121 represents an essential extension of the International Hydrographic Organisation S-100 for the administration of the marine domain.
In December 2016, the International Hydrographic Organisation (IHO) distributed the initial document describing the standard at the United Nations headquarters in New York. This meeting began the international effort to define the core features of the S-121 standard.
There are two goals of the S-121 standard, first: to have an open, international, coordinate-based, representation of maritime boundaries and their associated rights, restrictions, and responsibilities (RRRs) that are authoritative and easy to interpret. The second goal is to facilitate the deposit of maritime boundary claims with the Division for Ocean Affairs and the Law of the Sea (DOALOS) in a spatial coordinate-based format that fulfils legal requirements under UNCLOS.