This counts instances where the reserved field specifically in the SIP3 trailer, as opposed to the header covered by lportSmdsCntSip3RsvdInvalids, was found to contain a non-zero or otherwise invalid value. An admin treats this the same way as the header reserved-field counter, as a marker of a non-conformant sender or corrupted trailer, useful when trying to determine whether corruption is affecting the header, trailer, or both ends of a PDU. For example, seeing both the header and trailer reserved-field counters increment together for the same traffic stream suggests broad corruption of the whole PDU rather than a narrow, isolated field issue.