Hmm... I see!
Hey, Commander Anna! I couldn't help but overhear you there. What are you reading?
Oh, this? It's a Combat Manual. Combat Manuals contain records of Heroes' feats and the skills they learned, so they're really handy for when you want to inherit those skills.
I see... So they've got everything you need to know about Heroes, huh?
...Ooh, do they say anything about what their favorite snacks are?!
Before I answer that, maybe we should define what "everything you need to know" means...
Introducing Combat Manuals
Combat Manuals contain the knowledge of allies who have been sent home. They can be used as the source for actions like Inherit Skill or Merge Allies. Bear in mind, however, that when you create a Combat Manual from an ally, that ally will no longer be available to you as a unit. To create a Combat Manual, select Manage Barracks from the Allies menu.
Make Your Heroes More Manageable!
You can have up to 9,999 Combat Manuals per ★ rating for each Hero. Combat Manuals won't take up space in your barracks, so if you're holding on to a lot of allies to use for inheriting skills or merging, you can replace them with Combat Manuals to free up space.
A Couple of Precautions
Combat Manuals won't record some details, like inherited skills or the number of times an ally has been merged, so bear this in mind if you want to replace an ally who's undergone these processes with a Combat Manual. Additionally, once an ally has been replaced with a Combat Manual, you won't be able to bring them back.
Useful for Unlocking Potential, Too!
You can also use Combat Manuals in Unlock Potential. This enables you to do things like raising an ally's Combat Manual to a 5★ rating, then merging the Combat Manual with a 5★ ally.