This read-only integer is the ONU interface sequence number that, together with onuLlidDiid and onuOldVlan, forms the index into the EPON ONU VLAN translation table, identifying a specific interface on a given ONU. Admins use it to distinguish between multiple interfaces on the same ONU when configuring per-interface VLAN translation. For example, an ONU with several Ethernet ports would have a distinct sequence number per port, letting the admin apply different VLAN translations to each port on that ONU.