What does x4 bandwidth mean on a PCIe x16 slot?

What Should to Do With Extra PCIe Slots? pcie 2 slot In ASUS 「PCIe x16 Slots - 2」 pcie 30 in 40 slot List of Motherboards with 2 PCI-Express x16 Slots: ASUS TUF Gaming B760M-BTF WiFi, ASUS TUF Gaming Z790-BTF WiFi, MSI B760M Project Zero, GIGABYTE Z790

pcie x 16 slot It is actually +115% While gpu itself has only +70% - so other 30-40% is should be and P67 or sandy bridge pcie controller And this is