Wikipedia:WikiProject Geographical coordinates/region:

The  parameter specifies the political region for terrestrial coordinates. It is used to select appropriate map resources. If no  parameter is provided, GeoHack attempts to determine the region from the coordinates.

The region should be supplied as either a two character ISO 3166-1 alpha-2 country code or an ISO 3166-2 region code.

Examples of ISO 3166-1 alpha-2 codes: • AQ Antarctica

• AU Australia

• BR Brazil

• DE Germany

• GB United Kingdom

• HK Hong Kong

• IN India

• LK Sri Lanka

• RU Russia

• US United States

Examples of ISO 3166-2 region codes: • DE-TH Thuringia, Germany

• GB-BIR Birmingham, England

• NO-03 Oslo, Norway

• US-NY New York state, US

The oceans have the following Wiki assigned code elements per de:Vorlage:Coordinate.
 * XN Arctic Ocean
 * XA Atlantic Ocean
 * XI Indian Ocean
 * XP Pacific Ocean
 * XS Southern Ocean

In addition, two Wiki assigned code elements can be used with coord:
 * XZ for objects in or above international waters (similar to UN/LOCODE).
 * ZZ for use in examples.