NetBackup complete deletion
So this kind of idea is not stands only for NetBackup and may be used for Backup Exec too. The case is to make ability completely delete data from the volume. Now deletion (expiration) is not delete actual backup, it is possible to recover it using special utilitis. To make such kind of restore impossible, you need to generate a random data and write it on the volume, than do this again and again. Such functionality very useful for companies how need to replace volumes for some reason.