Sorry, just realized it is not a term used in this part of the world. It is the part you sit on and can remove and the back area which can also be removed.
If you go to:
Picasa Web Albums - CrystalFire - 2008 Shoddy Sofa
It seems mind boggling that I didn't see that vast expanse of white when the 2 very efficient and quick delivery blokes were here. - All bits of white are where the stitching or zipper is faulty and the inner padding is exposed. Each item was plastic wrapped. I thought it was the course of wisdom to photograph each fault that I found.
I will follow your advice and let you know of the outcome.