This read-only TimeStamp is described as the last time this SA was used, i.e. when traffic last passed under it. An admin checks this to identify SAs that are technically still up but have gone idle, which can help decide whether an idle-timeout policy should be tightened. For example, an admin cleaning up stale dialup sessions would look for SAs whose last-used time is far in the past despite the SA itself not yet having expired.