For example if you had a 5 image collage you wanted to create a Mat layer on top of. It would be very easy to do. A Ctrl+Click on the First image layer mask followed by a Shift+Ctrl on the 2nd, 3rd, 4th and 5th. To select all the images the Invert the selection to have a mask for the mat layer. Add a solid fill layer on top with this selection active the add a layer style.