This is the sub-port, or breakout lane, identifier for this LAN uplink when the physical port has been split into multiple aggregated sub-ports. On a QSFP port broken out into four 10G lanes, this field distinguishes which specific lane this row's data describes, since portId alone would otherwise point at the whole physical connector rather than one lane of it. A value of 1 alongside portId 3 would mean this row describes the first breakout lane of the QSFP port at position 3.