NetBackup keeps a record of where every backup is on every tape, and manages expiry of backups. If you write to a tape but don't fill it, eject it, and some time later inject it, NetBackup will write the next backups beyond the last unexpired image - or if the all the images (aka backups) have expired then NetBackup will begin writing at the start of tape again.
If you're talking about taking media from one environment to another, then that's a different ball game. As the second environment has no knowledge of the previous environment.
N.B. Never simply insert a tape that you want to read into a new environment without first at least write protecting it - or creating a media entry and assigning to a pool that isn't used for any backups. If you insert a tape that want to read, with no write protection, into a new environment that i sbusy writing backups, then it is highly likely that your tape will be overwritten IF the new environment auto assigns media to the scratch pool.
Anyway, to read a tape in a.n.other environment you'll need to injext the media, and in the new environment run the two import phases before any data can be restored.
