What are PCIe Slots in PC?

The QNAP one looks like it is x4 so I would need a ribbon cable and find a way to mount without fitting it in the x1 slot

MXH950 and MXH952 PCIe FireFly Optical NTB, Host & Target Adapter Adapter for IO, Server and workstation connections PCI Express x16 host port

pciex4 slot PCIe stands for Peripheral Component Interconnect Express, and it is a high-speed interface standard used for connecting various internal components in a

pcie slot bracket Slot 2, PCI Express x16 ; Slot 3, Slot not available on this board ; Slot 4, Slot not available on this board ; Slot 5, PCI Express x8 in a