Retimer card
The following shows the cable connections for the 8 x 2.5-inch NVMe configuration with one retimer card.
From To
Backplane 1: NVMe 0-1, 2-3 Onboard: PCIe 1, PCIe 2
Backplane 1: NVMe 4-5, 6-7 Retimer card on PCIe slot 1: C0, C1
Connections between connectors: 
1  ↔  1 ,  2  ↔  2 ,  3  ↔  3 , ...  n  ↔  n
Figure 54. Cable routing for the 8 x 2.5-inch NVMe configuration with one retimer card
Chapter 3. Internal cable routing 111