I have the same problem with the selection of test cases in a session: we see all test cases of all projects.

J'ai le même problème avec la séléctions des cas de tests dans une session : nous voyons tous les cas de tests de l'ensemble des projets.

In "Requirements' you just need to select a project so that the products are sorted by project.

I agree. By selecting the "Project" in all activities, it works. But in this case I can not.
I reproduced my problem in the demo available on your website. The project concerned is "Développement 002" and the product "Antenne rad".
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Je suis d'accord. En sélectionnant le "Projet" dans toutes les activités, celà fonctione. Mais, dans ce cas je n'y arrive pas.
J'ai reproduis mon problème dans la démo disponible sur votre site internet. Le projet concerné est "Développement 002" et le produit "Antenne rad".

Firt, one point that may be confusing is that PRoject in not really directly linked to project.
The relation always comes through project version : you sould link project to product version.
Adding product to project is an accelerator that will automatically link every version of product to the project. But at least one version must exist...

Then, when you know that, you can see that after saving the requirement, product list is empty : this is expected behavior.
Moreover, if you do not select project in project selector, when you create a riquirement and "change" the project, the product list is correct.

What is missing, is that when you select a project in the project selector (and so don't have to change project when creating the requirement), the project list is not correct.
This is an issue recorded as Ticket #2517.