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To enable custom alphas for certain objects, enable the Custom Alpha channel checkbox in the Channels panel and check the open the Custom Alpha Channels Editor.
Enabling the export of Custom Alpha channels in Studio
In this editor, you can define the different alpha channels you want to get (left column), set a name to identify each channel and assigning the object or group of objects to be included in the channel (dragging the objects into the right column). This assignment is not exclusive, so any object can be in more than one alpha groups.
Creating different alpha groups for the car body, the wheels and the whole car object