What exactly does the spec “PCIe x16 slot ” mean?

x16 to x4 PCIe bifuration riser cable PCIEX16E-Q4S x4 slot It's a slot of x16 size but electrically x4 This x4d slot Ultra Durable™ SMD PCIe x4 Connector The first PCIe x4 Slot supports the latest 25110 form factor Reinforced PCIe connectors

ocean king slot 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