This PEM UPI Cable gets installed when you want to expand the Socket count from 2 to 4. This will also double the DIMM count from 24 to 48 DIMMS. These Cables are also used in a 2 Socket connection as well. Below are some related parts.

Processor Expansion Module (PEM): 79PCJ
UPI Cables: 53J26
UPI Clock Cable: MXX5P
Power Distribution Module: 92CG2