This PerfCounter64 counter reports a count of bytes transmitted on the flow, in the downstream (network-to-subscriber) direction in the downstream (network-to-subscriber) direction, tallied over the most recent completed bin. An admin uses it for precise usage accounting on one specific subscriber flow, separate from the aggregate totals on the physical port it rides. If cmFlowHistoryBytesOutN2A on flow entry 28 totals roughly 308 MB for the interval while the flow's rate-limited counterpart is far lower, an operator can tell how much of that subscriber's offered traffic is actually being shaped away rather than delivered.