It's just a way to accommodate giving you cards in small cases.
For the unused PCI slot, I won't look at dell.com documentation but note I've seen such used for the phone modem. Maybe something else but all this is documented. I didn't look it up but pulled from memory.
The computer has 2 PCI slots in the removable card cage.
I use 1 slot for my ethernet adapter, the other is empty.
The card cage riser connects to the motherboard.
The rear card cage plate covers are horizontal.
Directly beside the the card cage inside the computer appears to be another unused PCI slot on the motherboard with a rear verticle cover plate.
What is this setup ?
All the specs list only 2 PCI slots.
It almost seems like 2 out of 3 ain't bad music.
Ps. I gave up on getting an explantation from Dell.