This string is the serial number of the hardware component underlying this internal link, as tracked on the Fabric Interconnect side. It matters on the same internal chassis backplane link, but the record kept on the Fabric Interconnect (switch) side rather than the IOM/server side because the serial number ties this specific switch-side record to a physical, replaceable component when opening a Cisco TAC case. An admin escalating a recurring link fault on this link's fabric-interconnect side would quote this serial number to identify the exact fabric interconnect hardware involved.