Bonsoir Emmanuel,
Merci pour le pointeur vers cette page.
La phrase clé est"The elevation of a level is the height of its floor on which furniture is placed. But the bottom of the walls of a level is at the elevation of that level minus its floor thickness to join nicely with walls at lower levels when you use default values.
The elevation of a level may also be negative to design a basement or foundations."
J'ai pu me debrouiller !