Special/Extract Mask
 
 

The Extract Mask operation creates a new texture, which contains the Mask. If you have set a Source Texture, no new texture will be created and the mask will be added to the source texture instead. The function is useful to create Opacity Maps, which limit an operation to e.g., a rectangular area even if you are using the pen tool.

The selected Mask Mode defines the outcome of the function. As you can see in Image 2, the flood fill tool was applied to the red flower. Some ellipses were drawn to get Image 3.