The Subtract from Selection command converts the layer mask of the active layer into a selection, which is subtracted from the selection that is already active in the image. White areas of the layer mask are selected, black areas are not selected, and gray areas are converted into feathered selections. The layer mask itself is not modified by this command.

Figure 10.108.  Illustration of Subtract Layer Mask from Selection

From left to right: the original image with a selection, the Layers Dialog with a layer mask created with the “Layer's alpha channel” option, after “Subtract from Selection”.