Calculations

Calculations uses two Channels to make a more refined selection, which can be output as a new Channel or a selection.
Source 1 layer can be imagined as actually being on top of the Source 2 layer.
Opacity determined the opacity of the top layer, so by setting it to zero the bottom layer is revealed.
Blending modes Add and Subtract are only available in Calculations, and the Offset and Scale are only visible when these options are selected.
Offset goes from -255 to +255 and is the amount of lightening or darkening applied to the result.
Scale can be between 1 and 2.
Clicking on Mask reveals the panel below it.