This Integer32 is described as the session key lifetime in bytes used by this Dynamic Crypto Map, forcing a rekey once that much data has passed under the current key regardless of elapsed time. An admin uses this byte-based limit as a companion to the time-based limit so high-throughput tunnels rekey based on actual usage, not just the clock. For example, an admin on a high-bandwidth backbone tunnel might set this to a smaller kilobyte-equivalent value so keys rotate well before the time-based limit would otherwise trigger.