Ok so first of all, somewhere in the local game files there is your stored death notes.
While in game and looking at a deathnote (even someone elses) you have a save and a load option.
Pressing save will immediately download the deathnote to your computer.
Pressing load will give up a submenu with all of your deathnotes. The submenu is sorted by recency with a sub-sub section for stared DN’s.
What this means is that you can quickly switch between multiple different deathnotes mid game. So for example if you are possessor you could copy the DN of the knight you possessed for a knight and then quickly switch back to your possessor DN when you are trying to be fancy or whatever.
The submenu can be accessed out of game in order to favorate or delete certain deathnotes for whatever reason