Exactly as described, this counts packets received on this benchmark port that were shorter than 64 octets yet were otherwise well-formed with a valid FCS. Legitimate Ethernet frames can't be shorter than 64 bytes, so a nonzero and climbing count here during a test most often indicates the remote generator or an intermediate device on the path is producing malformed short frames rather than any standard test pattern. For example, seeing this counter increase specifically while running an EMIX sequence that includes a custom small frame size below 64 bytes would point to a misconfigured custom frame-size setting rather than a link problem.