While the MaterialID and ObjectID channels provide a multicolored material / object silhouette to be used as selection mask in a 2D image editor for further adjustments, non pure red, green, blue or white can cause halos in the selection edges when doing the chroma color extraction.
The Custom Alpha feature generates an individual pure black&white Alpha mask per each individual object you choose, providing a perfect selection mask for that object.
To enable custom alphas for certain objects, enable the Custom Alpha channel checkbox in the Channels panel and check the