Add to Object Buffer
Use this page to specify up to 12 object Buffer IDs for the object. These object channels correspond to the object channels defined in the Multi-Pass setting's Channel menu.
Let's say you activate a Compositing tag in the Channel tab for a given object and assign it ID 1. If you subsequently define an object channel in the Multi-Pass settings with an ID of 1, an alpha channel will be created that matches the object. This method can be used to create any number of alpha channels.
An ID of 1 can also be assigned to other objects, letting you combine a wide variety of objects that can be output as alpha channels. Refer to the Multi-Pass chapter for more details.