The Mask node allows for the use of multiple textures to control opacity. 


Mask

Figure 1: Mask with layer group Node


Mask Parameters

Add/Remove Layer - Adds or removes composite texture layer pins.

Enabled - Whether this layer is applied or skipped.

Attach to layer - When selected, this layer modifies the input of the next lower layer that is not itself attached to a layer. Otherwise, it applies to the output of the next lower layer.

Pass through - In pass-through mode, this is a non-isolated compositing group. Its background is the output of the layers below it. When disabled, this is an isolated compositing group. Its background is black and fully transparent.

Source - Which value from each pixel to use as the mask.