Create a four-layer structure in the Inventory like samples "chassisRackVSphere-ui"

In vsphere-client-sdk/Samples chassisRackVSphere-ui,the hierarchy is "Chassis App-Chassis-host"(ingnore the rack here),just like the pics showed below.I have read the sourse and the plugin.xml in the project.The hosts is releated to the chassis in plugin.xml using an extension point and in the java service adapter initialize the host with chass.

Question Here ↓

So if I want to create a four-layer structure ,just like  "Chassis App-Chassis domain-Chassis-host" ,how should I set the plugin.xml?How should I associate the "Chassis Domain" with the layer?