This reports the byte rate this interface is transmitting, expressed per second rather than as a bin total. Because it's a rate rather than a cumulative total, this is the figure to compare directly against this port's negotiated line rate to see whether outbound traffic is nearing saturation, which the bin's cumulative TxBytes counter for the current, still-accumulating short interval bin cannot tell you by itself. For example, TxBytesPerSec reading 118500000 bytes/sec on a port provisioned for a 1 Gbps Ethernet service means the egress side of that circuit is effectively pegged at line rate.