Starting with NBU 4.5 MP3, NetBackup started to return tapes back to Scratch automatically.
In 6.x, the functionality is controlled by EMM.
Have a look at output from ' nbemmcmd -listsettings -machinename <master>' Look for the line listed below towards end of output.
This is output from my lab server with no modifications:
RETURN_UNASSIGNED_MEDIA_TO_SCRATCH_POOL="yes"
You can change this with nbemmcmd -changesetting ..........
Admin Guide I also says the following about Scratch pool:
A scratch pool is a special volume pool from which NetBackup can transfer volumes to another volume pool when that pool has no volumes available. When the images on a volume expire, NetBackup returns it to the scratch volume pool if it was assigned from the scratch pool.