The interposer cable connects the storage expansion node to the compute node.
The storage expansion node comes with an interposer cable that is connected to the system board. The free end of the cable exits the storage expansion node near the left-rear corner. After you attach a compute node to the expansion shelf, you connect the free end of the interposer cable to the interposer connector on the compute node system board to establish the connection between the storage expansion node and the compute node.