Most of the SUD trade-union practice a syndicalism of struggle (syndicalisme de lutte), like factions of the CGT, FO and the CNT.

Most of these were unions who had refused to choose a side in the 1946 split between the reformist Workers' Force and the communist CGT.

The CFDT expelled several unions from the health and the post and telecommunications federations located in the Paris region because they supported wildcat strikes self-organized by the workers.

The expelled post and telecommunication unions decided to create a new federation, Sud-PTT, which got encouraging electoral results a few months later.

SUD unions are now important in the health care sector, education, transportation (including the SNCF and Air France), cultural institutions such as the Louvre, as well as in some companies such as Michelin, Thales, or even students with Solidaires étudiant-e-s, syndicat de luttes.