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

What are PCIe Slots in PC? pci e x16 slot Slot power ; x1 cards are limited to A at +12 V and 10 W combined ; x4 and wider cards are limited to A at +12 V and 25 W combined ; A full- pcie 4 in pcie 3 slot ssd Product Overview · This PCIe x1 to PCIe x16 Slot Extension adapter provides enhanced compatibility between motherboards and a variety of PCI-Express expansion

pcie 40 in pcie 30 slot So in the first scenario, both physical x16 slots only provide 8 lanes, in the second scenario the first x16 slot provides 16 lanes (for full