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

MSI® HOW-TO Install graphics card on PCIe x16 slot with butterfly lock pciex16 slot pcie x16 slot, pcie x1 slot, pcie x4 slot, pcie x8 slot, which pcie slot to use, does it matter which pciex16 slot configuration Notice that the PCIe x16 slot has a retention lever Figure PCIe adapter removal Figure Motherboard with PCIe and PCI slots Tech Tip

santa wonderland slot Why does a PCIe slot which has only 4 lanes physically has 16-slot size? What difference does sharing secondary PCIe x16 with other PCIe lanes