Reference Cinema 4D Basic Features BodyPaint 3D Layer Menu
Function available in CINEMA 4D Prime, Visualize, Broadcast, Studio & BodyPaint 3D

New Alpha Channel

If you choose this command, a new alpha channel is created for the material channel or texture based on an existing selection. If there is no selection, the alpha channel is filled with white and the entire channel will be visible.

You can also use this command to save selections. To restore a saved selection, activate the alpha channel and choose Select / Selection From Layer from the main menu.