The PCI subsystem page allows you to separately configure the three 16x PCI-E slots to maximize compatibility with older cards. When left in their default Auto setting they will be PCIe 3.0/2.0/1.0 compatible and while this should be good for most PCIe devices, if you have an older PCI-E 2.0 or 1.0 device you can hard-set any or all slots to act accordingly.