Wow, I hate to contradict the Mighty Bob, but I do believe he's incorrect on this question.
If you have a Catalog backup configured in your Vault Profile it will pick either: A) A random volume from the defined volume pool (if there are any) or B) a random tape from the Scratch pool (if there are no volumes in the defined volume pool).
I definitely recommend using a specificly defined (and populated!) volume pool for these tapes as even when they expire (if they go to Scratch), they have 'Catalog data' on them and NBU won't write over that type of data (you have to Label the tape to clear it for use in other jobs) and it will then Freeze the media.
$0.02