This read-only integer is the ONU interface sequence number that, together with onuAggLlidDiid and onuAggDstVlan, indexes a row in the ONU VLAN aggregation table, identifying which specific interface on the ONU the aggregation rule applies to. Admins use it to target VLAN aggregation configuration at the correct physical or logical port on a multi-port ONU. For example, an ONU with multiple Ethernet ports would need a distinct sequence number per port to apply separate VLAN aggregation rules to each.