Hi Pierre:
This is correct.
To add SECTION A above some chapter names, and presumably, SECTION B above others—a perfectly reasonable request—Fm needs to pull the words out of the document. You will need to add the words SECTION A someplace before Chapter 1 (or at the top of Chapter 1) and assign it a unique paragraph style so that Fm can pull the words into the TOC/TDM. If you want to add the section heads to the table of contents, but not to the page, you still have to add them to the page, but you can make them white or very small. Or both. That way they won't be visible to the reader on the page, but they will still be available to FrameMaker to call into the TOC/TDM.
Bob is also correct in his reply of course, but he is referring to something you are going to ask us soon—adding a title to the top of the TDM that likely says Table des Matières. This is also addressed on the list, as number 6.

When are ready to add the title, you will have add it at the very top of the generated text. To be allowed to keep it after you update your book:
- Table des Matières must be the first paragraph in the flow, and
- Its style name cannot end in the suffix for that generated file.
For example, you could name it the French equivalent of Contents, as per Bob or ContentsTitle as per my post, but not ContentsTitleTDM because TDM is the suffix for a table of contents in your French version of FrameMaker. If you add TDM to the end of the style name, Fm will remove it when you update your book.
So many rules!
~Barb