Overwriting and Appending to tapes

Hi, I am using BE 2014. I have two tapes in my autoloader. For tape A the "Overwrite Protected Until" column says 25 Jan 2015 and for the tape B it is 20 Jan 2015. For both the tapes the "Appendable Until" column says Not appendable. When I run a backup job now the backup is done to tape B. My concern is once the tape B gets full what will happen to the backup jobs. Are they going to overwrite the contents on tape B or use tape A? I do not want to overwrite the contents on tape B as I am taking one time backups which I want to store for life time.

Any documents which explains how the tapes are used?