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authorities managing contributions to social benefit funds
Explanation: Your context refers to "instances sociales" which covers all those bodies collecting and managing funds from which benefits are paid. It extends to include health, illness, death, retirement, unemployment, professional training.
WHilst employment/unemployment are concerned, "employment authorities" omits the idea that these authorites are in charges of handling contributions and benefits.
The UNSA, the Union Nationale des Syndicats Autonomes, is a national organisation of independent unions.
In this text, they refer to « instances sociales » and give a couple of examples. Towards the end of the document :
« A sa demande, nous avons rappelé au ministre les points forts de nos dossiers, et précisé les conséquences qui en découleront en termes de participation effective de l’UNSA aux différentes instances sociales (OPCA, Caisses de retraite) et ministérielles. »
OPCA’s are organizations with official authorization to collect funds for professional training
They are sector specific, by which I mean that architects, translators and surveyors will contribute to one of a number for liberal professions, ditto the hotel and catering industry.
The other example given is of “caisse de retraite” retirement pension fund.
This confirms the reading of “instances sociales” to cover bodies such as the (UK) NIC, retirement funds, health funds, educational funds whatever. Part of the dreaded “charges sociales”.
The translation workplace 
