This value is the configured port mode for this internal backplane link, describing how the port is provisioned to operate (e.g. as an access-style link versus a trunk carrying multiple VLANs). 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 the configured mode has to agree with how the connected blade's traffic is actually tagged, or frames intended for a specific VLAN can be silently dropped. Checking mode on this backplane link is one of the first steps an admin takes when a specific VLAN's traffic isn't reaching a blade even though the link itself shows up.