Adobe Photoshop CC 2014 v.14.xx - Manual (Page 367)

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Because higher pixel values represent lighter colors, adding channels with overlapping pixels lightens the image. Black areas in both channels
remain black (0 + 0 = 0). White in either channel results in white (255 + any value = 255 or greater).
Add mode divides the sum of the pixel values by the Scale amount, and then adds the Offset value to the sum. For example, to find the average of
the pixels in two channels, add them, divide by 2, and enter no Offset value.
The Scale factor may be any number between 1.000 and 2.000. Entering a higher Scale value darkens the image.
The Offset value lets you lighten or darken the pixels in the destination channel by any brightness value between +255 and –255. Negative values
darken the image; positive values lighten the image.
Subtract
Subtracts the pixel values in the source channel from the corresponding pixels in the target channel. As with Add mode, the result is then divided
by the Scale factor and added to the Offset value.
The Scale factor may be any number between 1.000 and 2.000. The Offset value lets you lighten or darken the pixels in the destination channel
by any brightness value between +255 and –255.
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