When backupExec looks at the tapes in the partition, of the job is Append, it will verify that the tapes belong to the prpoper media set
If the job is Overwrite, BackupExec will grab the "oldesttape that it can see (that is, the one that has been in Overwritable status longest), taking into account the Global Option to use Overwriteable tapes in the media set first, or Scratch tapes first
When you load the tapes into the partition, you can re-lable then starting with slot 5, then 6 etc. This will gurantee that slot 5 is the oldest tape, and the rest should be used in slot order