If you were to, say, play around with mods for a few hours but want to go back to earning achievements, your only option is to load from your original John files, losing any progress you made with mods installed. Instead of saving to the regular John section of your save menu, it will create a John section. You install a mod, play around on John for a little while, and save the game. Example, say your character's name is John. When you install a mod and load a save, any saves made will be separated into a new save slot.
