IBM System p Series Cabling Manual page 38

After you finish
__ Did you receive a cable hook with your rack shipment?
v No. Proceed to the next item.
v Yes. The cable hook can help manage the server cables in the back of the rack. To install the cable hook, slide
it into the slots located on the back of the rack as shown in the following figure:
__ Return to your initial server setup checklist and complete the next step.
See the back views of the model to see the slots and connectors.
Cabling a model 9405-520 or 9406-520 and a Thin Console
Learn how to connect the Thin Console, power cords, external cables, and the optional electronic
customer support cable, cable the expansion units, attach devices, install the PCI Cryptographic
Coprocessor card, route the cables, and start the server.
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System i and System p: Cabling your server