How do I make save file slots?

I want to make selectable save file slots, I already made the menu with the icons, now I just need some way to store data.

I want my game to have Super Metroid's or Castlevania SOTN's file select screen where you see which ones been played on and which ones are empty.

How can I load that kind of info into the game? I suppose the save/load system isn't the correct method.

You would use the system save/load actions to do the actual saving and loading, but to determine if a save slot is in use you would use extra data like variables saved into local storage and depending on their value, the player has saved the game or is yet to use it. You can also save other data like the player name or score or number of collected items to display on the save slot screen.

Local storage function was what I needed. Thanks! :D