Creo Topology Optimization > Optimization Setup > Topology Regions > About Topology Regions
  
About Topology Regions
Use Topology Region to define regions that you optimize by removing material, and regions that are kept intact. The topology design region (TPROP) is where the optimization process includes elements in, or excludes elements from, the optimization. For example, if a volume interfaces with other components, you would not want to remove material from this geometry.
You can define a topology region using either components or volume regions. While defining topology regions, you can select regions to exclude from the topology optimization. While you define topology regions, you can optionally impose fabrication constraints to the regions.
In an assembly, you can optimize the topology of only one single component. Therefore, all the references for the topology regions that are included in an optimization study must belong to the same component.