HOW IT WORKS
Subworld Library does a LOT of code injection, as Terraria was not made with subworlds in mind.
Subworlds are highly customisable; from how big or small they are, to what ModSystems can update inside of them and even how they are lit.
Subworld Library removes Space, both Oceans and the Underworld from subworlds, allowing them to be extremely small without issues.
Loading a subworld is straightforward. Loading screens can be as simple as text on a plain background, or something complex, like an item selection menu.
Subworlds save to a directory named after the main world. A subworld and/or changes to players inside it can be temporary. Deleting a world deletes all of its subworlds as well.
Subworld Library works in Multiplayer with little to no extra work required from modders. A server is opened for every subworld being occupied.