The Union of Nations is the main international organisation in Multipolarity II. It has a very broad scope, and borders on being a world government as much as an international organisation.
The Union government is composed as follows.
The Legislative Branch is composed of the Upper House, the Security Council, which is all player nations, all major NPCs, and all corporate city-states. A measure must pass the UNSC to go onto the General Assembly, the Lower House. The General Assembly is akin to the US House of Represenatives, with 500 seats: 1 representative per UN member; all remaining seats are allotted based on population. The Assembly gives power to those with clients or Philanthropy's special ability.
Acting as the nominal executive of the Union is the General Secretary, chosen by sortition, who can serve non-consecutive terms. The Secretary's primary function is the ability to veto resolutions, with the veto, similar to the US President's, requiring two-thirds to override.
The Supreme Court of the Union of Nations, or SCOTUON, is the judicial branch that has evolved outside the formal framework of the Constitution and exists to clarify ambiguities and close loopholes. It has GM protection, being the voice of the GM effectively, and became a major player in response to Sonereal and Double A's trolling of the Union.
The Union controlled 80 provinces at the start of the game, the bulk of them canal zones, granting the Union and enormous amount of revenue to use. The 80 provinces are grouped into "Zones," represented by "Zone Leaders," who are chosen by the mostly autonomous Zones. Zones can hold just about any government or legal structure they desire, provided it is in line with UN statutes.
Per statute, the UN was authorised to control its own military, raising, training and equipping them. They cannot leave UN territory without the support of the Security Council, however. These armed forces were known as the United Nations Peacekeeper Corps, or UNPC.
Several agencies evolved in international law. INTERPOL (later the UN Intelligence Committee, renamed by Thorvald of Lym so he could take the name INTERPOL for his own organisation) was the black ops force of the UN, primarily used to search for terrorist cells in consenting countries. The UNPC is mentioned above. The World Bank received 1/4 of the UN's revenue and distributed it based on requests, and was overseen by three Chairmen with veto power. The International Monetary Fund was also given 1/4 of the UN's money, and was tasked with eliminating poverty around the world.
Similar to the US government, besides very powerful Constitutional provisions, day to day legislation took the form of Statutes. Statutes did such things as establishing the Peacekeepers, creating international laws, or providing for the establishment of the Olympic Games. Statutes could be readily passed and repealed due to requiring a mere simple majority.
A full list of UN Statutes can be found here.