This writable object selects which firmware partition should become active on the next boot, accepting 0 for no change, 1 or 2 to pick a specific partition, or 3 to move to the next partition in sequence. It's the operational control an admin uses to switch the device's boot target between its redundant firmware images without necessarily triggering an immediate update. An admin who has just staged a new image on partition 2 would set this object to 2 to make that partition the next-boot target.