This read-only Counter64 reports how much the undersize frames (runts) count increased since the previous stats-polling interval for this interface's live, currently-active error-statistics record, i.e. the per-interval rate rather than the lifetime total. A nonzero value here means new failures of this specific kind are occurring right now on this link, not just accumulated history, since on this specific link these are typically collision fragments or the product of a malfunctioning NIC on whatever host is attached upstream, rather than a fault local to this interface. For example, a cluster of undersize frames on this port right after a firmware update on the attached host's NIC is a common real-world cause, which this delta field would show as a sudden nonzero reading in a single interval.