According to Directive 2014/67/EU of the European Parliament, registration may already be mandatory even for short-term assembly or project assignments within Europe. All countries have implemented this regulation and there are regular updates in the reporting procedures and the functionality of the portals.
Which national social security law is applicable to temporary agency workers working across borders? The European Court of Justice (ECJ) has now dealt with this explosive question (judgment of 03.06.2021, ref. C-784/19).