This object identifies the specific DSL port, not just the ONU as a whole, from which a rejected channel-change request came, narrowing the fault down to one subscriber line. Combined with rejectedOnuId, this lets a technician isolate whether a channel-change failure is limited to one customer's port (wrong entitlement or a maxMulticastStreams cap reached) or affects the ONU or channel more broadly. A single port index showing up repeatedly as rejectedCustomerPortId would point a technician straight at that one subscriber's DSL line rather than the shared multicast infrastructure.