Masks enable you to hide and reveal parts of a layer and to apply special effects with precision. Masks can be created from selections, from images, from the luminance values of images, and from scratch.
When you move a selection, some of the pixels surrounding the border are included, especially when the selection has been antialiased or feathered. Matting cleans up the border by removing these pixels.

First create a mask from source luminance with the actual image.

Then create mask from selection.