This preserves the count of Phase-2 exchanges sent by an expired tunnel that were rejected by the peer specifically because they referenced security parameters the peer didn't recognize, per its description. It's the historical, more specific cousin of a generic reject counter, pointing to SPI or SA-reference desync rather than a plain policy mismatch. A history record showing this counter climbing steadily right before the tunnel's expiry suggests an SPI mismatch was the proximate cause of that tunnel's failure.