A media set in Backup Exec is a set of instructions for how the software handles appending and if/when the media will be available to be over written.

The default media set in Backup Exec is set to keep data indefinitely.  This is good to get you up and running quickly, but is not what 99% of users will keep. 

I suggest creating a new Media Set and naming it something to help you remember how you set it up.  Daily/Weekly/Monthly are very common names to help remind you how you setup the overwrite protection.  One note to remember on this is that the locked period STARTS when the current backup FINISHES.   This means if your backup starts at the default 11 PM time and finishes at 8 AM the next morning, and you have the Media Set setup preventing an overwrite for a full day after the backup ended –  your media could not be overwritten until after 8 am the following day.

This is to help protect the information stored on the backups for compliance and data retention.

If you need to use the same tape a week later, and you are doing nightly backups, then you should set the overwrite protection to 6 days.  This allows the media to become over writable again the morning that it will be re-used.

When you setup a backup job you will choose which media set you would like the job to use.  The recommended practice for this is to get the new media setup in the Scratch Media set and let Backup Exec automatically move that media over to the selected media set when it is used in the backup for the first time.

On the flip side if there is a media that you are needing to re-use for some reason but it is currently still in the overwrite protected period you can move that one media to the scratch media set and make it available for re-use.