The Cluster & Uncluster Tree buttons group trees and plants (two types of objects) of the same variety as if they were one object.
A world project is often easier to manage when trees and plants have been clustered together in the same layer. The cluster and uncluster buttons consolidate groups of trees according to the chunk that they occupy. For example, if clustered, all Small Oak trees in a chunk would be treated as one object rather than ten distinct ones.
When trees in a layer are unclustered they are often confusingly renamed in the format "Game_Object_0". This can make it hard to recognise which kind of tree or plant they are in the layers window. The easiest way to find out is to right click their name and select "Find in render panel".
|“||We don’t have any way to group objects together other than trees, so yes, it is better to think in terms of chunks when it comes to tree placement, following the guideline to try not to use more than four tree/plant types per chunk. When trees are grouped, each group counts as an object. If one tree type is across a chunk line and there are no other trees of that type within the chunk, the lone tree will be counted as a separate object. By moving it into the other chunk, it then becomes part of the group and will reduce the tree object count by one. It is really hard not to exceed four types per chunk, but with this goal in mind, it helps keep the number reasonable!||”|
— Rebecca, Sims 3 World Building Team, (source)
|World Description | Layers | Terrain Paints | Roads | Objects | Effects | Lots|
|Cluster Trees | Edit In Game | Brushes | Chunks | Routing|