Yes, the filon or tin siding stops at the floor level, and on the outside of the rig you'll see a horizontal trim piece that runs along the siding. Below that is the "J Wrap", as MFL pointed out it looks like a J if you look at a cross section of it, because it curves in and slightly under the rig. It's on the whole rig, slides and all. On a class A this lower section also has the baggage doors in it, because that storage is below the floor of the rig.
Mondooker, if you had a single J panel and you needed it for either: A) A rig that is mostly built, that you've already spent labor and materials building but haven't gotten paid for, but finishing it with this panel would allow you to sell the rig and get paid, or B) a rig that you've already shipped and gotten paid for, where do you use the panel?
You're correct, it IS a conscious choice where parts go, and sales trumps service any day of the week and in any industry, not just RVs. Bean counters call the shots nowadays.