Riser with Three PCIe Gen3 FH slots One x16 and 2 x8

What is PCIe x16 pcie slot x16 I tried the clear CMOS procedure again and that didn't fix the issue I tried a GTX 750 ti(that uses the power of the slot and not external pcie 4 ssd in pcie 3 slot Some motherboards feature multiple PCI Express x16 slots, which are typically used for adding graphics

pcie 1 slot PCIe x16 Gen expansion backplane with two slots The PCIe x16 Gen3 backplane has two PCI Express x16 slots, with one slot supporting the OSS target