Adobe Photoshop CS3 - Manual (Page 284)

PHOTOSHOP CS3
User Guide
277
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.