This is the 64-bit version of cefPrefixBytes, avoiding the wraparound risk the 32-bit byte counter faces on a high-throughput prefix. Byte counters wrap even faster than packet counters on high-bandwidth links since each packet contributes many bytes, so this 64-bit object is essential for accurate accounting on any prefix carrying more than a modest amount of traffic over a polling interval longer than a few minutes. A prefix sustaining 2 Gbps of traffic would overflow a 32-bit byte counter in well under a minute, making this 64-bit counter the only practical option for that destination's accounting.