Posting of workers

ln recent years, the Court of Justice of the European Union bas handed down many judgments on the posting of workers and the social security scheme to which they are affiliated. Among various important questions, one crucial matter was to determine the scope of the certificates issued by the social security bodies of States of origin, which stipulate that workers are still affiliated to their national social security scheme when regularly posted in other Member States. Commonly referred to as "ElOl certificates" under Regulation 1408/71, on the coordination of social security schemes, or later as "A 1 certificates" under Regulation 883/2004, to what extent are these certificates binding on the authorities of host Member States challenging the legality of posting operations?