French to English Law/Patents Automotive / Cars & Trucks public bus seating system

I need to distinguish between the terms "siège" ,"assise" and "embase" for a patent describing seating on a bus. "Siège" is the seat as a whole, and I see that "assise" is usually translated as "seat" i.e. the part of a chair that you sit on, as opposed to the back. I don't really want to use "seat" for both terms.
I thought of "seat base" but then there is also the "embase", the base attaching the seat to the floor of the bus, which I've translated as "base", so "seat base" might cause confusion.
Any ideas? thanks!