The upper slot holds a x16 PCI Express graphic card. The "x16" here simply represents a combination of 16 logical PCI Express lanes. By blocking some contacts with tape it is possible to get the card to run at slower modes.

Retail motherboards generally cannot be operated at slower PCIe modes, because Intel did not provide for this option (though they are common in AGP-based systems.) Yet, this can be achieved by nicely asking a motherboard manufacturer to provide a suitable BIOS version, and then blocking all unnecessary pins with tape as mentioned above. This is a pretty simple way to test slower transfer rates.

Shown above is a x16 PCI Express slot as well as the contacts that have been taped to achieve a specific transfer mode. Click to enlarge.