This numeric field is the chassis identifier associated with this endpoint's addressing. For an Ethernet LAN uplink port, which lives on the fabric interconnect itself rather than in a blade chassis, this value is not a meaningful chassis location and is expected to sit at its baseline of 0, unlike the same field on a server-facing port where it points at a real chassis number. You would expect to see 0 here for Fi-A/1/3, whereas the analogous field on a blade server's vNIC endpoint might read 3 or 4.