Cluster generation uses the settings from the individual HLOD Levels to decide how it should group Static Mesh Actors in the scene together. This generation process can take some time depending on the settings use, the number of Actors being groups, whether materials are generated, and generally on your hardware’s specs.
Once you’ve setup your specific settings for the individual HLOD levels you need, click the Generate Clusters button.
Once the process starts you’ll see a progress bar appear that indicates the LOD level being generated.
Now that the process has completed you’ll see the HLOD Outliner populated with all the clustered Actors.
You can expand the individual LODActors to see what Static Meshes make up this cluster by clicking the arrow button to the left of the name.
You can visualize the clusters in the level by selecting a LODActor from the HLOD Outliner and locating it in the editor viewport.
To visualize how the generated clusters work in the editor you can use the Forced Viewing Level slider to see the HLOD in action without the need to have it transition at a specific screen size. This is helpful for troubleshooting any issues appearing on screen that may be part of the generated cluster.
Should you run into any issues you don’t like with the generated cluster you can expand the specified cluster and select the offending Static Mesh Actor. You can then click and drag it to another cluster, or you have the option to Remove or Exclude it from the generation of the cluster by right-clicking on the Actor's name in the list.
Actors can also be excluded on a per-instance basis by selecting them in the level and in their Details Panel set the option for Enable Auto LODGeneration to false.
Further, if you wish to add an Actor to a Cluster you can click and drag from the World Outliner to the Cluster you wish it to be included in.
Repeat this process while adjusting the values in the HLOD's level Cluster Generation Settings until you are happy with the clusters being generated, then continue on to the next section: Generate Proxy Meshes.