This is the default lifesize in kilobytes assigned to an IPsec SA as a global policy, per its description, unless overridden in a specific cryptomap definition - meaning an SA will force a rekey once this much data has passed through it, regardless of elapsed time. It matters for high-throughput links where the time-based lifetime alone might never be reached before the volume-based limit forces a rekey first. A global value of 4,608,000 KB on a saturated 100 Mbps tunnel could trigger volume-based rekeys well before the time-based lifetime ever would.