This read-only Counter64 reports the number of bytes written to a given storage device since the Synology NAS last booted, as a 64-bit counter that avoids the wraparound issues of the 32-bit spaceIONWritten on high-throughput or long-uptime systems. Admins prefer this counter for accurate long-term write-volume tracking on busy NAS units where the 32-bit counter could wrap between polling intervals. For example, an admin building a monthly write-volume report for capacity planning would use this 64-bit counter rather than spaceIONWritten to avoid undercounting from a wrap.