The overwrite protection period is a nice add-on that prevents accidental destruction of existing backups within a given time frame.
The append period defines for how long the user may add data to an existing backup set. A good example would be a weekly backup, which is used for a complete working week - and locked away after that. All future backups would have to be stored in a new backup set.
slide show: Tandberg 420LTO and 220LTO Streamers