French to English Tech/Engineering Construction / Civil Engineering Quantity surveying standa

Please note this is from a BELGIAN document (standard) which contains a great many very specific Belgian usages! I am only proofing it!

There is no proper context to help define what is meant by this term, which appears in various places in connection with formwork, etc., though the juxtaposition with other items seems purely fortuitous; it seems to refer to some kind of poured concrete floor, and in fact, I suspect it refer to the underside of the poured slab; it often appears alongside 'plancher nervuré' ('ribbed floor'???!!)

I half suspect it is referring to a plancher 'à caissons' — like a coffered ceiling; these are common in this industry sector, and would be consistent with the discussion of special formwork for these and the 'planchers nervurés'. Would anyone know if this is a specificall Belgian usage meaning the same thing?

The only Google results I have found for the term used by the original translator, 'cassette floor' always bring me back to removable, modular fooring systems for use as ad hoc dance-floors in marquees etc., making me think the original translation is off the mark.

Does anyone have any idea what it refers to, and what might be the translation?

I've tried usual resources like GDT, IATE, Termium etc., but even on Google there are precious few meaningful hits — and I've not found any so far that actually helped with translating it.