How does NetBackup sequence/prioritize scratch tape re-use

Question about scratch tape re-use - Does anyone know what process NetBackup uses to determine scratch tape use? The sequence of prioritizing re-using tapes?

I had always assumed that scratch tapes were FIFO - as tapes expired and went into the scratch pool they would get re-used in some sort of order. I assumed it was first in, first out.

I am trying to grab and remove a specific barcode tape range, so as they expire and go into the scratch pool, I move them into another pool so they are not used.

For example, I have tapes with barcodes starting with 0 and tapes starting with 1. I have over 400 scratch tapes starting with 0, I got 30 back, 10 were ones I needed to move. When I came in this morning, one of these new scratch tapes was in use.

It does not have a low mount count, so that is not it.

Maybe based on last use time?

Thanks!