Соответствующая норма
Yugoslavia, Federal Republic of
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section A. Establishment of open towns
According to the Report on the Practice of the Federal Republic of Yugoslavia, “the opinio iuris and the customary nature of rules relevant to the establishment of these zones [open towns and undefended places] in FRY is absolutely clear”. 
Report on the Practice of the Federal Republic of Yugoslavia, 1997, Chapter 1.8.