A very important decision you will need to make is whether you would like the Axes of your World to be endless or fixed. Fortunately, you can choose a separate option for each axis on your World!
A Fixed Axis has a set boundary. If a World Region load operation or external source attempts to add a user for a World Cell that sits beyond this boundary, the Add User requests is ignored. The result is that the Streamable Grid has only a single set of Cells present on the axis.
An Endless Axis has no set boundary, and World Region load operations or external sources are able to add users for World Cells that sit beyond the axis boundary of the Streamable Grids. Whenever a World Cell beyond the original Streamable Grid's boundary needs to be loaded, the World loads a copy of the World Cell. Put another way, all Cells on the Streamable Grid for the given axis are repeated in both directions. The effect is repeated each time the player gets close to the World's boundaries (defined by the Bounding Box of all Zones on the World), which is where the endless term comes from. This creates the illusion of a spherical world, because the Player is able to travel continuously in a single direction with the World repeating itself over and over again.
When using an endless axis, you can identify whether a World Cell is a part of the original fixed portion of the Streamable Grid or whether it is part of a "repeated" Streamable Grid by using its CellOnEndlessGrid property. The value of this property will match the value of the World Cell's CellOnStreamableGrid property only when the World Cell has not been repeated.
When referencing World Cells and/or storing them in key based collections, you must always use the CellOnEndlessGrid to identify the World Cell. This is because multiple World Cell's with the same CellOnStreamableGrid value may be loaded at the same time, but each World Cell will always have a unique CellOnEndlessGrid value!
The Use Endless Layers option is only used if the Predominant Axes Type of your World is Three Dimensional.
If the Predominant Axes Type is not Three Dimensional, the Use Endless Layers setting will be ignored, and the World will always using an Origin Cell Layer value of 1 (even if you try to set it to a different value).
This means if you want to have a World that can repeat endlessly on the Layer axis, you must ensure that all Streamable Grids use Three Dimensional Axes!
Conceptually, World's repeat at the border of the first and last Zone on a given axis, however in reality they actually repeat at the edges of the World Grid Cells that contain those borders. For example, if you observe the World Map below, you'll notice that the left border of the South America Zone is to the right of the left edge of the map, while the right edge of the Australia border is to the left of the right edge of the map.
In this example, if the World Map were repeated to the right of Australia, the South America zone border would not touch the Australia Zone border. Instead, the World Grid Cell that contains the South America Zone's left edge would be placed adjacent to the World Grid Cell containing the right edge of Australia's Zone border.
When dealing with islands or other water locked landmasses, this is generally not an issue, as it will just result in some amount of water sitting between the repeated World instances. However, if the edges of these Zones are connected by land and it's necessary for one Zone to start repeating exactly where the other Zone ends, you will have to be careful about configuring your World Grid Dimensions and/or Zone Positions such that the Zone edges that are connected align with the borders of the World Grid Cells!
You can also artificially add more spacing between repeated World instances by using the Gap Settings in the General Settings tab of the World. Gap Cells allow you to insert additional World Grid Cells between repeated World instances, in cases where without the Gap Cells the Zones along the edge of the repeated World Instances would be too close together.