This read-only Gauge32, in units of bytes, gives the average frame size (transmitted and received combined) for this row of the mlb basic-load history table. A falling average suggests the MAC address (MacIndex) is sending mostly small frames such as acknowledgements or keepalives, while a rising average suggests bulk transfers, which helps an admin interpret what kind of traffic is driving the Frames and Bytes counters in the same row. An admin puzzled by a high Bytes count but a low Frames count for a row would check mlbFrameSize and see it confirms the traffic is dominated by large, near-MTU-sized frames rather than many small ones.