Cell
A Cell is a data structure and the representation of somewhere in a Worldspace, it can be either a piece of the world, such as a piece of terrain, or an interior, such as a house or a city.
A Cell is always contained inside a Worldspace, and the worldspace is made up of cells.
You can view a Cell as a piece of world, as the player moves around, cells are loaded and unloaded from the game, allowing you to have a huge and continuous world.
Every cell is either an Interior Cell or an Exterior Cell:
Interior Cell
An Interior Cell is a cell that represents something that is an Interior, such as a City in the world, or an house. It is something that doesn't have to be split, and is loadable as a whole, without having to worry about performances, NPCs count or anything else. An Interior cell is often accessible from an Exterior Cell, but you can have nested Interiors too.
An Interior Cell is always contained into a Worldspace flagged as type: Interior.
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Exterior Cell
An Exterior cell represents a piece of a much bigger space, that will be loaded only if the Player is nearby that location.
Usually you will have to create an Exterior cell to compose a world that is not loadable as a whole, because it's too big, too many NPCs would be in it and so it is better if it is streamed as the player walks by.
So in other words an Exterior Cell is nothing else than tile of a bigger world, that is connected to other tiles and that will be loaded only if the player gets nearby.
An Exterior Cell is always contained into a Worldspace flagged as type: Interior.
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