What are the top uses of PCIe x1 Slots?

Will PCIe x1 x4 cards work in x16 slot? can i put pcie x4 in x16 slot 7PCIe slots Up to300Wauxiliary power One PCIe x4 gen 3 Apple IO card Four PCIe x8 gen 4 Two single height Two double height Two PCIe x16 gen 4 Double can i put ddr3 in ddr4 slot PCIe UD Slot X： PCIe x16 slot with 10X strength for graphics card Ultra-Fast Storage：3* slots, including 1* PCIe x4

candy 99 slot What is PCI Express? · PCI Express x1 — Single lane, smallest slot · PCI Express x4 — Four lanes, second-smallest slot · PCI Express x8 — Eight lanes, second-