On 27 April 2017, the Court of Justice of the European Union here decided on the binding effect of an E101 certificate (now called A1 form), which aims at facilitating the freedom of movements and services.
The Court of Justice judged that this binding effect is applicable even if the competent institutions or the courts of the Host Member State believe that the conditions under which the worker concerned carries out his activities do not correspond to the conditions for being subject to the social security of the other Member State. However, the competent institutions of the Member State which issued the A1 form are expected to reconsider whether it was properly issued and, if appropriate, to withdraw it if challenged by the competent institution of the Member State in which the employee actually works. The competent institution of the host Member State may have doubts as to the accuracy of the facts upon which said certificate is based and the information contained therein because the information does not match the requirements of Article 14(2)(a) of Regulation No 1408/71 here.
On the basis of the ruling, Belgian institutions are bound by the A1 form issued by the competent institution of any other Member State. Belgian institutions cannot disregard this form and subject the workers to Belgian social security, so long as this form is not withdrawn or declared invalid by the Member State that issued it.
Considering the binding effect of the A1 form, you should make sure that the workers you post to another EU Member State, or who are working in several Member States at the same time, get their A1 form.