basically, if you create a copy of balatro.exe and rename it, it acts as a fresh copy of the game, with its own save directory that has the same name as the exe file and erverything. When selecting your game directory, you can't choose which specific exe file it should look for, or which save data folder, meaning it can only look at your main instance.
The reason I use this method is because it makes it easier to switch between different versions of steamodded and running different mods.
