It means that:
If you use AMD Crossfire (running two identical AMD GPU at the same time) and installing both card on PCIe_E2 and PCIe_E4, PCIe lane width will fall back to PCIe 2.0 x4 mode on both slot (quarter the speed of a real x16 PCIe slot).
Or it also means that:
Both slot (E2 and E4) are...