On Symantec Backup Exec, I see there is a Media set with 26 Weeks Overwrite protection period and 1 year appendable period. On all the help information I was able to find the appendable period is usually smaller than the overwrite period.
Backup Exec "protects" media from being overwritten with the "Overwrite Protection Period". So, in your question, a media will not be permitted to be overwritten until 26 weeks have elapsed since the completion of the last write operation on that media.
Because you might want to continue to append to a media after the initial write (but not actually overwrite what's already there), Backup Exec tracks a separate "Appendable" period. For 1 year after the last write operation completes on a given media, in your question, additional data can be appeneded to the media.
Bear in mind that any appending causes additional writes to the media, and any future checks as to whether the media is able to be appended-to or overwritten will be based on that last write time.
Typically, where you see this used is in libraries where you have, say, multiple tape cartridges loaded and you want to do "one tape per day" type rotations. Let's imagine a library with 5 tapes in it and a once-per-weekday backup rotation. You'd put the media into a set with an "Overwrite Protection Period" of 6 days.
You load the library with tapes "1" thru "5", all of which are brand new and are overwritable.
You can imagine how this could apply to multiple sets of medias (rotating an entire week in and out of the library, etc) with a little mental extrapolation.
The append period is a bit more involved. Take the same scenario above, but say that you run an "incremental" backup during the day each day, Tuesday thru Saturday at noon, and want that incremental backup to be appended to the tape used for the prior night's full backup. You'd set an append period of, say, 20 hours.
There's a crash course in Backup Exec media protection. >smile<
My guess is that:
The of the backup/media set as a file. To do the backup the first time it will write all data to that file. To save disk and processing time, it only adds changes to those original files to the backup set, it does not actually change the backup media.
This means that if someone creates a file and then deletes it, you can still restore it from the backup set.