This mode is essentially 'Easy mode', you can save the game whenever you want and respawn an infinite amount of times. Press F5 to qicksave. SCPs that are Safe class are easy to contain and are fully understood, this does not mean that they are not harmful.
In this mode, you cannot save in random places, you can only save at certain locations. Euclid class SCPs are not very well understood and require a substantial amount of resources to contain, they are also usually harmful. Essentially 'normal' or 'medium' difficulty.
This is basically the game's 'hard' mode. In this mode you cannot save and death is permanent, SCPs are also more agressive. Keter class SCPs are anamolies that are hard to contain and escape containment, usually these SCPs are extremely harmful.
This mode is a custom mode where the player can choose which mode they want with some extra things, they can change the agression of SCPs, permanent death and how available saving is.