This records the frame size in bytes the PDV (jitter) results in this row were measured at, since packet delay variation, like latency, is characterized separately across the standard RFC2544 frame-size sweep. An admin uses this to know which point on the sweep a given jitter number belongs to, since larger frames can introduce more variable queuing delay behind them than smaller frames on a congested link. For example, PDV rising sharply only at larger frame sizes recorded in this field would point to buffer or scheduling behavior specific to bigger frames rather than a general jitter problem.