What is the role of the mask option in the "Apply Image" and "Calculate" commands of PS?

Mask can be regarded as a transparent protected area of a certain shape, which covers the protected area on the image. floor

Mask is generally divided into display layer and hidden layer.

The image command can be used to blend the current image with the layers or channels of the target image in an image blending mode.

You can also add another image as a mask for the current image.

The calculation command can synthesize two channels in the image, or it can keep the synthesis structure of two channels.

Save to a new image file, or directly convert the composite structure to the selected area.