The docs seem to indicate that if you have a directory structure (say over NFS share) you want to backup to tape, you must first back it up to a datastore (read: local storage on the PBS), that you cannot backup the files straight from a directory structure you must first store them locally in the datastore?
Yes, normally tape is a second level backup for the worst case and long term archival.
Tapes normally are not kept on site, to ensure that even a destruction of the office/datacenter/... like, for example, a fire does not make one lose all data.
With PBS this also allows to leverage the whole thing in a more flexible, efficient and powerful manner, as one can do tape-backups async, the deduplication layer can be used so that subsequent backups only need to add the delta of new backup chunks from the content addressable storage (note, the backups themself are still full backups, and one can configure a schedule for when a new media set needs to be created), besides that backing up file-level to tape is normally pretty slow compared to writing out big chunks that are ordered by physical disk location.
