This is the MAC address associated with this internal backplane link's interface. It matters on the internal Ethernet link that carries a blade's traffic from the chassis I/O Module up to a Fabric Interconnect over the backplane because it is the Layer 2 address UCS Manager and the fabric interconnect use to identify traffic sourced from this specific internal port, useful when tracing a MAC seen in a switch's forwarding table back to a physical chassis link. A MAC of 58:8D:09:70:42:D9 on this backplane link is what an admin would search for in a fabric interconnect's MAC address table to confirm this specific internal link is the one forwarding a suspect flow.