About Selecting the Active Model in the Layer Tree
You can select the active layer model from the:
• Drop-down list
• Model Tree
• Graphic Window
In Part mode, the active model is the part itself. You do not need to identify it before working with layers.
In Assembly mode, you may select the active model before working with layers.
In Assembly mode, if you are in the process of modifying a part, subassembly, or skeleton (that is, you have selected a sub-model and click Activate in the shortcut menu to set it as the active model), the Active Object is automatically set to the model being modified.