Part I Their respective definitions：
ISO (International Organization for Standardization) is a non-governmental, non-profit international standards organization. ISO was founded in 1947. China is a formal member of ISO. The national organization representing China in ISO is the National Standardization Administration (managed by the State Administration for Market Regulation). ISO is responsible for standardization activities in most areas of the world today (including military, petroleum, shipbuilding and other monopoly industries). ISO's highest authority is the annual "General Assembly", and its daily office is the Central Secretariat, located in Geneva, Switzerland.
IEC 
(International Electrotechnical Commission) is a non-governmental, non-profit international standards organization. It is a non-governmental international organization that formulates and publishes international electrical and electronic standards. It was formally established in London, England in 1906. China is a permanent member of the IEC and participates in the work of the IEC in the name of the Standardization Administration of China.
ITU (International Telecommunication Union) is an international standards organization that exists as a United Nations agency. It is also the oldest international organization among the United Nations agencies, with its headquarters in Geneva, Switzerland. China has been a member of the ITU Council since 1972, and currently the Ministry of Industry and Information Technology represents my country in the ITU.
ITU (International Telecommunication Union) is an international standards organization that exists as a United Nations agency. It is also the oldest international organization among the United Nations agencies, with its headquarters in Geneva, Switzerland. China has been a member of the ITU Council since 1972, and currently the Ministry of Industry and Information Technology represents my country in the ITU.
Part II: Respective Purposes：
ISO's purpose:
To promote the development of standardization and related activities throughout the world, so as to facilitate international material exchange and services, and to expand cooperation in the fields of knowledge, science, technology and economy. The main tasks are: to formulate, publish and promote international standards; to coordinate standardization work worldwide; to organize information exchange among member countries and technical committees; to study relevant standardization issues with other international organizations.
Development history: ISO's predecessor was the International Standards Association (ISA), which was formally established in 1928 by the standard organizations of seven countries including the United States, the United Kingdom and Canada. The outbreak of World War II forced ISA to stop working. After the war, there was a lot of work to be done, and international standardization also ushered in new development opportunities. In October 1946, representatives of 25 national standard organizations held a conference at the Institution of Civil Engineers in London and decided to establish a new standardization organization based on the original ISA, dedicated to "facilitating international cooperation and the unification of industry standards." The conference adopted ISO's first charter and rules of procedure. On February 23, 1947, the International Organization for Standardization (ISO) was formally established.
IEC's purpose:
To promote international cooperation in standardization and related matters (such as certification) in the field of electrical and electronic engineering, and to enhance mutual understanding between countries.
Development history: Compared with ISO, IEC has a longer history. Since the electrical field involved in IEC is the product of the deep development of the industrial revolution and the protagonist of early international trade, at the six international electrotechnical conferences held from 1887 to 1900, the participating professionals unanimously agreed that it was necessary to establish a permanent international electrotechnical standardization organization to solve the global electricity safety and electrical product standardization problems. In 1904, the International Electrotechnical Conference held in St. Louis, USA, passed a resolution on the establishment of a permanent organization. In June 1906, representatives from 13 countries gathered in London to draft the IEC Charter and Rules of Procedure, and formally established the International Electrotechnical Commission, with the purpose of promoting international cooperation on all issues related to electrical standardization in the fields of electrical, electronic and related technologies. In 1947, it was incorporated into ISO as an electrical department, and in 1976 it was separated from ISO and has remained independent to this day.
The purpose of ITU:
To maintain and expand international cooperation to improve and rationally use various telecommunication means; to promote the development and application of technical facilities to improve the efficiency of telecommunication services; to study, formulate and publish international telecommunication standards and promote their application; to coordinate the behavior of various countries in the field of telecommunications, and to promote and provide assistance to developing countries.
ITU development history: The history of ITU can be traced back to 1865. In order to smoothly realize international telegraph communication, on May 17, 1865, representatives from 20 European countries including France, Germany, Russia, and Italy signed the "International Telegraph Convention" in Paris, and the International Telegraph Union (International Telegraph Union) was announced. In 1906, representatives from 27 countries including Germany, Britain, France, the United States, and Japan signed the "International Radiotelegraph Convention" in Berlin. In 1932, representatives from more than 70 countries held a meeting in Madrid, Spain, to merge the "International Telegraph Convention" with the "International Radiotelegraph Convention", formulate the "International Telecommunication Convention", and decided to officially change its name to the International Telecommunication Union (International Telecommunication Union) from January 1, 1934. The Telecommunication Standards Bureau was established within the ITU to establish national standards related to communications. With the approval of the United Nations, the International Telecommunication Union became a specialized agency of the United Nations on October 15, 1947.
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