ICAO DOC 7300
Article 37 - Adoption of international standards and procedures:

Each contracting State undertakes to collaborate in securing the highest practicable degree of uniformity in regulations standards, procedures, and organizatiog in relation to aircraft, personnel, airways and auxiliary services in all matters in which such uniformity will facilitate and improve air navigation.

To this end the International Civil Aviation Organization shall adopt and amend from time to time, as may be necessary, international standards and recommended practices and procedures dealing with:

a) Communications systems and air navigation aids, including ground marking;
b) Characteristics of airports and landing areas;
c) Rules of the air and air traffic control practices;
d) Licensing of operating and mechanical personnel;
e) Airworthiness of aircraft;
f) Registration and identification of aircraft;
(...)