From my understanding, this document should only be asked for persons who have already a permanent goverment employee position in France. For applications for ATER positions, this would for instance be the case for an academic that would have succeeded at the agrégation, which is one of the recruitment level for high school teachers. A number of persons in academia in France start by getting the agrégation or a lower-level CAPES guarenteed high school employment before starting their PhD. They could then apply for an ATER temporary position, but the administration will need to be told about it beforehand.
This document mentions that it is only required in such situations (I - POUR LES FONCTIONNAIRES DE CATEGORIE A which means for A-level public servants.)
If you are not familiar with the French system, it is quite likely that you do not need this document.