This Integer32 (0 or 256..2147483647 kilobytes) is described as the number of kilobytes transferred before the SA is completely deleted, with 0 meaning this lifetime is withheld from proposals sent to the peer. An admin sets this to force full SA renegotiation once a data-volume threshold is crossed, which matters for high-throughput tunnels where the byte count, not elapsed time, is the more meaningful renewal trigger. For example, an admin on a tunnel carrying nightly bulk backup traffic might lower this value so the SA renegotiates partway through a large transfer rather than staying on one key for the whole job.