Does the notion of international secondment have a social function? This is the starting point of this study. The aim is to demonstrate that the notion of international secondment, as it emerges from international law, has both a relative and an absolute function, which is none other than that of protecting industrial relations and the domestic public policy of the State of secondment. Putting forward the idea of international secondment as a functional notion makes up for the impossibility of definition raised by the doctrine of the second period (Julien Icard, Etienne Pataut, Agnes Cerf-Hollender). Neither the Rome Convention of June 19, 1980 on the law applicable to contractual obligations, nor the Rome I Regulation (EC) no. 593/2008 of the European Parliament and of the Council of June 17, 2008 on the law applicable to contractual obligations, including the rules of private international law applicable to posted employees, define the posted worker or international posting. The functional dimension of international secondment rules out the idea of an anti-law concept, of an equation between fraud and international secondment.