cpqDaTapeDrvMagSize reports the number of tape slots in the autoloader magazine currently loaded into a tape drive, returning -1 for non-autoloader devices. This tells an admin how much unattended backup capacity a specific autoloader currently has available before someone needs to swap in a new magazine. For example, an admin planning an unattended weekend backup job would check this magazine size to confirm enough tape slots are loaded to cover the expected data volume.