Understanding how NetBackup writes to tapes and what happens when images expire will help you better manage your tapes.
Tapes are linear - meaning that they write from the front to the back.
if we look at a tape.
now the day1image expires on the tape - the assigned date does not change and you have this.
if you try to write to this tape again it will ONLY write in the blankspace as tapes are Linear - meaning they can only append (tapes cannot write here and there like a disk can)
Now day2image expires - the assigned date of the tape goes blank - no assigned date
Now look at it with multiplexing.
you use the tape - it gets an assigned date.
if the backup for server1 fails or if the backup image expires then you have this