To guarantee that your islands run on all supported platforms, Fortnite has some memory limitations in place.
With the previous limits in Creative, you had to ensure that your experiences didn't go over the 100,000 memory units allowed on a regular-sized island.

Choosing an XL island would get around this limit as long as each area did not cross the 100,000 unit threshold.
With Unreal Editor for Fortnite (UEFN), you can use expanded memory capabilities to make larger and more diverse experiences. To fully take advantage of this, it helps to understand the way that memory use is calculated in UEFN.
Introducing World Partition
World Partition is the magic behind building a large island experience. This feature automatically divides the world into a grid, and streams only the necessary cells.

World Partition uses Streaming, which loads and unloads cells, and Hierarchical Level of Detail (HLOD), that groups and decreases the amount of detail on assets as the player camera moves away.
For more information on World Partition, Streaming, and HLODs, see Streaming and HLODs.
The New Session Layout
The Cell Memory Used bar that Fortnite creators are accustomed to has changed. As long as Streaming is enabled in your project, you are no longer limited to a maximum amount of memory units on your island.
When you load into your session, you will see a Current Memory Usage bar on your HUD.
Current Memory Usage

Current Memory Usage shows a maximum of 100,000 memory units to give you a picture of the local memory usage on your island.
The main difference in your Creative Edit session is that memory is now calculated based on player positions on the island rather than the total memory used by all the assets on the island. Performing a memory calculation will give you the most accurate memory consumption values for each streaming cell.
Some assets will stay in memory regardless of the player's position, and will add to the memory usage anywhere in the map.
Assets that count toward total memory currently include but are not limited to:
- Devices
- Custom landscapes
- Custom assets
Most other assets will load and unload as explained above, and the bar will show the memory usage for a specific area of the island.
Note that if any area in your level exceeds the 100,000 mark, you will not be able to publish your island.
As a UEFN user, you can exceed the 100,000 thermometer limit as long as you don't publish. Keep in mind that people collaborating with you on a console might hit a hardware limit of available memory that will kick them out of the session. They can reconnect once the issue is resolved.
Memory Calculation
Your island needs to be able to run on all hardware platforms for you to successfully publish.
Before publishing, it is vital to make sure your project does not exceed memory limits. From the Project dropdown, select Launch Memory Calculation.

This process calculates the memory used in every cell.

This will take several minutes to complete. During this process, you do not have control of your player character. You cannot cancel this flythrough, so strap in!
The top left corner displays this HUD status widget during the calculation:

Once finished, the player character will teleport back to a spawn pad and you’ll see one of two statuses:

or

You will also see a popup giving the memory calculation result. Pressing Continue will return control of your player character.

You can now fly around your island to see the amount of space different areas take up.
Notice how the Memory Used bar changes during the flythrough. You can use this to identify the areas in the level that might be over budget.
If your island is over budget, you will need to fix the local memory issues before publishing.
My Island Is Over Budget. What Can I Do?
There are many strategies to save on memory once you hit the limit. Try any combination of the following:
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After the memory calculation, fly though your island, look to see where the thermometer reaches critical memory limits, and delete actors.
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Turn Streaming ON in the World Partition panel. Although this is a must for large islands, smaller islands may also require streaming, especially if the content gets too dense.
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Reduce the number of devices your island is using. Creative devices aren’t streamed out: they will add to the memory of whatever cell the character is located on.
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Take a look at the Project Size window. If you see some memory-heavy objects on disk, chances are they’ll also be heavy on memory at runtime. To check your project size, click the Project dropdown and select Project Size.
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Reduce the quality level on custom asset LODs, especially on those assets that are only visible from a distance.
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Reuse the same mesh multiple times. In a forest made of 100 trees, use 5 variations and duplicate them around instead of using 100 unique trees. This will make a huge difference on the memory footprint.
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If using HLODs, generate them throughout the process of the project and ensure that you're only generating them for objects you can see from far away.