The French do this because when building a building of several storeys they are apparently in some doubt as to where a "storey" starts and finishes. IOW, does a given storey include the floor slab of the floor above or not?

So, if you are on the 2nd floor, the "ceiling" of that floor is the plancher haut 2ème étage. Seems to me we'd call it the floor of the 3rd storey, or the third floor .... However, talking about "floors" gets complicated, depending on US or UK usage, whether what one might call the ground floor is above a basement, etc.

I would probably call your plancher haut sous sol the "ground-floor slab" but "entry-level floor slab" might be a safe option (assuming there are not entrances worthy of the name at a basement level or off an elevated concourse, etc.).

It basically means the ceiling of the basement and the floor slab of the floor/storey above.