This Counter64 tracks the number of IPsec Security Associations that are at the end of their life, meaning they are being phased out (typically due to key/lifetime expiry) before final removal. Admins monitor this to catch tunnels that are about to be rekeyed or torn down, which can help correlate transient packet loss with SA churn. For instance, a spike to 10 dying SAs right before a scheduled rekey interval would be expected, but a spike at other times could point to instability.