This records the frame size in bytes that the latency results in this row were measured at, since RFC2544 requires latency to be characterized across the standard frame-size sweep rather than at just one size. An admin uses this to line up a latency figure with the correct point on the sweep, because store-and-forward latency naturally increases with frame size on some architectures, so a rising latency trend as this value increases could be entirely expected rather than a fault. For example, seeing similar latency at 64 and 1518 bytes on a cut-through switch fabric would be a normal, expected result rather than evidence of a problem.