This is a cumulative counter of bytes transmitted out this channel's interface, counting only bytes actually sent across the physical medium and excluding framing overhead. An admin polls it over time to measure per-channel transmit throughput within a bonded Inverse Multiplexer group and spot channels that are underutilized or idle. For example, a channel whose byte count stops incrementing while its siblings keep climbing suggests that link has silently stopped carrying its share of the bonded traffic.