Both NetBackup and Backup Exec use proprietary tape formats. I know that current versions of NetBackup can read the Backup Exec tape format, but AFAIK the converse (Backup Exec reading the NetBackup tape format) doesn't work.
I've only had a bit of experience with Backup Exec on Linux, so I'll leave my comments at: It backed-up the files we needed reliably. We never did a disaster recovery of any of the Linux machines that were covered strictly from tape. We opted, instead, to reinstall their OS and then restore data files from backup where necessary.
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.
On Monday evening at 7PM the library chooses tape "1" (though it won't necessarily choose the numerically lowest number-- I'm just doing that to make the example easier) and writes to it. That tape is now overwrite-protected for 6 days-- 'til sometime after 7PM on the next Sunday.
On Tuesday evening at 7PM the library choose tape "2" and writes to it. That tape is now overwrite-protected until sometime late on the following Monday. The library can't choose tape "1" for this operation because it's overwrite protected.
This continues until Friday, when all the tapes in the library are overwrite protected except "5", forcing the library to choose that one.
When the following Monday's backup rolls around, the library is forced to use tape "1" because all the other tapes are still overwrite protected. From this time on, the tapes come off of overwrite protection less than 24 hours before they're actually needed, but no sooner.
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.
On Monday at 7PM the backup runs normally. Tape "1" is used and marked overwritable on the following Sunday evening. It remains appendable until late afterooon on Tuesday.
At noon on Tuesday the incremental backup runs. Since this is an append job, the library choose a tape that's "appendable". Because tape "1" is still appendable (the append period has expired for all the other tapes), the library chooses tape "1" and appends to it.
Tuesday night's full backup runs on tape "2", just like we'd expect.
Wedneday at noon, the incremental backup runs. Since tape "1" became non-appendable during Tuesday afternoon it cannot be used. The library chooses tape "2" since it became appendable after the completion of the Tuesday night backup.
There's a crash course in Backup Exec media protection. >smile<
Best Answer
The Append Period (APP) and the Overwrite Protection Period (OPP) perform two different tasks.
APP defines how long you are prepared to allow appending more jobs to a piece of media once you've started writing to it. It's analogous to asking, "For what time period am I prepared to allow backup sets to be written to the same piece of media"? Setting too long a time period may increase the risk in the event of the media failing. Setting too short a period may result in using too many media. APP is measured from when the media is allocated.
OPP defines for how long you want any backup set written to the media to be protected from overwriting. It's analogous to asking, "If I have a backup set, what's the minimum amount of time I want it to remain available"? Setting too short a time period may result in a backup set being overwritten before there is another, more recent, backup of that backup set. Setting too long a time period may result in you using too many media. OPP is measured from when the last backup set was written to the media.
The OPP and APP are reliant on your Media Protection settings being set to Full or Partial.
As an example, suppose you are doing full backups to tape 5 nights per week. You may wish to set your APP to 23 hours (so you don't reuse the same tape for more than one night) and the OPP to 6 days (so you will end up keeping backups on tape for around a week once you've taken the APP in to account).
They work together to create the media usage scheme. It's not understanding them that messes it up!
It means that you can't append more data to the end of the backup set. It will become overwritable when the OPP expires, or if you move the media to the Scratch Media set.