The HELCOM' ArcGIS REST API, short for Representational State Transfer, provides a simple, open web interface to ArcGIS. A major portion of the API, described in this section of the help, allows access to services hosted by ArcGIS Server.
All resources and operations exposed by the REST API are accessible through a hierarchy of endpoints or Uniform Resource Locators (URLs) for each GIS service published with ArcGIS Server. When using the ArcGIS Services portion of the REST API, you typically start from a well-known endpoint, which represents the server catalog. See the Resource hierarchy to learn about how the services in the REST API fit together under the catalog.