What does online/offline media actually mean?

Media in Backup Exec can be marked as being in one of three locations: Online; Offline; In a vault (though multiple vaults can be defined).

So how are online and offline media defined?

Backup Exec uses these locations to determine which media to use for backups. While vaults do not affect the media selection process, they can be used to mark which media are off site, which can then be used for reports to coordinate the movement of media off site, and also to reconcile the contents of offline vaults.

Online media is media which is immediately available to Backup Exec.
Offline media is media which is not immediately available to Backup Exec.
The other alternative is media stored in a vault.

For example:
A tape which is in the drive is online.
A tape which is not in the drive if offline.
Alternatively an alternative location (such as an off-site store) may be defined as a vault.

A backup-to-disk file which is still on the disk is online.
A backup-to-disk file which is not on the disk is offline.
Backup to disk files cannot be moved to a vault.