Well that is kind of a mouthful.
Quick technical tip for TSM users. When TSM is finished with a tape volume (virtual or physical) the default behaviour is to not relabel it, but instead to reuse it with the existing label (volume ID).
This happens to be not a very good thing if you are using a virtual tape library with deduplication. Why? Because virtual tape libraries don't reclaim space--at the block or segment level that is no longer referenced by valid backup data--on a volume that is still in use. Even if "in use" in this case means sitting in the (virtual) scratch pool doing nothing.
So, the good folks over at IBM figured this out, and have a way to force TSM relabel a volume when it is returned to the scratch pool. In terms, the VTL now sees that the data referenced by this volume is no longer valid, and can be freed up for reclamation, which will free up physical space on your deduplicated VTL. Good stuff.
In order to take advantage of this, you need to set "relabelscratch=yes" in your library definition. You can find additional detail here: http://www-01.ibm.com/support/docview.wss?uid=swg21330242. Note that if you are at TSM 220.127.116.11 you will need to apply a patch for this to work, if you are at 5.5.2.x and higher, it should work without further patching.