The collection is mostly intended to allow you to unlock the highest difficulties without playing the game twice. See the full list for further details.

I intend to expand the collection to other purposes over time.

Instructions

Disable steam cloud. Otherwise the game won't see the changed save. You should be able to re-enable at some point after using the save.

Right click the game in Steam -> properties -> uncheck steam cloud

You can renumber any save that has a number to protect existing saves. Otherwise, just copy into the directory as instructed below. System saves are safe to overwrite, other saves are not.

Steam Deck

Your directory will look more like this:

/share/Steam/steamapps/compatdata/1999770/pfx/drive_c/users/steamuser/My Documents/KoeiTecmo/

Games

Atelier Meruru DX

• Clear Save (with nothing on it). As close to a new game experience as I could make.
• Clear Save (good equipment and loads of cash)

Location: <path to steam installation>/Steam/userdata/<steam id>/936190/remote/SAVEDATA

This is intended for clearing the game for the best end without the need for NG+.

Atelier Shallie DX

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Shallie DX\SAVEDATA

Atelier Sophie DX

• Pre-Final Boss

Location: Documents\KoeiTecmo\Atelier Sophie DX\SAVEDATA

Atelier Firis

System saves unlock highest difficulty. The Clear saves have nothing on them and are for convenience. They allow you to have all costume effects active at once, rather than equipping each costume as you need it.

• System save (DX)
• Clear save (DX)
• System save (original)
• Clear save (original)

Location (DX): Documents\KoeiTecmo\Atelier Firis DX\SAVEDATA

Location (Original): Documents\KoeiTecmo\A18\SAVEDATA

Atelier Lydie & Suelle

System saves unlock highest difficulty.

• System save (DX)
• System save (original)

Location (DX): Documents\KoeiTecmo\Atelier Lydie and Suelle DX\SAVEDATA

Location (Original): Documents\KoeiTecmo\Atelier Lydie and Suelle\SAVEDATA

Atelier Lulua

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Lulua\SystemData

Atelier Ryza

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Ryza\SystemData

Atelier Ryza 2

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Ryza 2\SystemData

Atelier Sophie 2

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Sophie 2\SystemData

You're probably looking here for cosmetic stuff, but they seem to check ID to unlock that.

Atelier Ryza 3

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Ryza 3\SystemData

Atelier Marie Remake

System save unlocks highest difficulty.

• System save

Location: Documents\KoeiTecmo\Atelier Marie Remake\SystemData

Blue Reflection Second Light

• System Save
  • Location: Documents\BLUE REFLECTION Second Light\SystemData
• Clear Save
  • Location: Documents\BLUE REFLECTION Second Light\GameData00
  • Or whatever number you want
• NG+ Pre Final Boss Save
  • Location: Documents\BLUE REFLECTION Second Light\GameData00
• Be aware that these are all 'data.dat' files, be sure to not overwrite them with each other.

System Data unlocks hard/deathwish without the need for NG+. Clear Save will allow you to get the NG+ specific ending (changes are small, but you probably do want to see the NG version first) and all dates without playing the game twice. The NG+ Save takes you straight to where the story changes happen.