Yes, I know it and used it. How I can transfer a "digital" .sav to a physicial DS catridge?
For checkpoint I believe what you need to do is put the .sav file at
sdmc:/3ds/Checkpoint/saves/[TITLE ID] [GAME TITLE]/[ANY NAME]/[GAME TITLE].sav (where
[TITLE ID] is the 4 letter unique ID of the game,
[GAME TITLE] is the short title from the game's header, and
[ANY NAME] is... whatever you want).
The easiest way to check what all those names should be is to simply dump the current save in Checkpoint then find the one that looks right, usually the game title will be a shorted version of the actual name, though sometimes it's the Japanese title instead of the English one.
Then once you've put the save file there if you go into Checkpoint you should see the folder you made and be able to restore it to the cart.
If Checkpoint doesn't see your cart you can force it to reload by holding B for a couple seconds.