To load the transparency of a layer or the contents of a Layer Mask as a selection, Command (Mac) / Control (Win) -click on a layer or layer mask thumbnail in the Layers panel. You can then add, subtract or intersect the transparency mask of that layer (or any other layer) to that initial selection by right mouse clicking on the layer thumbnail, or with the following shortcuts:
• Command (Mac) / Control (Win) + Shift to add additional layer and or layer mask to the selection.
• Option-Command (Mac) / Alt-Control (Win) to subtract another layer and or layer mask from the selection.
• Option-Command (Mac) / Alt-Control (Win) + Shift to create the intersection of two layers and or layer masks.