This object holds the customer (C-)VLAN ID applied to a subscriber port's traffic under this EPON service profile, the inner tag that separates one customer's Ethernet frames from another's. Admins set it because the C-VLAN is what keeps broadcast and multicast domains isolated per subscriber even though many ONUs share the same physical PON fiber. For example, setting eponSrvProfilePortVlanCvlan to 201 tags that ONU port's LAN traffic with C-VLAN 201 so it stays segregated from a neighboring customer's traffic all the way to the aggregation switch.