Some puzzles come in a box with several puzzles. At first, the site only tracked the box not the individual puzzles and you would have to choose one puzzle to list the size and piece count.
Now, you can mark the box as a multi-pack and then after saving, you can attach individual puzzles to the box.
In your collection, this will show one entry for each puzzle plus an extra for the box. If someone tries to trade or buy, or if you just want to transfer, you will be working with the box not the individual puzzles. If you change the status on one puzzle, all the others will follow since they are all in the same box. So if you HAVE a multi-pack with 5 puzzles in it, and you switch the box or one of the individual puzzles to the status of I USE TO HAVE then all the others will switch at the same time.
If you really did lose one puzzle or you decided to burst the collection, then you would need to create new puzzle entries in the system to take the collection apart.