The Alpha Diff drawing mode

The Alpha Diff mode works essentially on the Alpha channel of the tool and the image pixels.
If the Alpha channel value of the pixel in the image is less than the Alpha channel value of the pixel of the tool, the new image pixel color gets the tool pixel color.
Otherwise, a percentage of the tool pixel Alpha channel value is computed in function of it and of the image pixel Alpha channel value. This percentage is applied on the image pixel color, and then, added to the tool pixel color.

This Alpha Diff drawing mode is very useful for drawing transparent colors and keeping the same "global" Alpha component value of the tool in the image. A good example is for a layer which would contain shadows. With this mode, it is possible to draw a shadow with different tones of grey but always keeping the same Alpha value.
We can see this mode like a global non-drying mode.

Let's see an example with the cloud above the house.
On the first images, three ellipses have been applied successively on the cloud layer with the default Color mode with different color values.

Original image

Color 255 0 0Color 128 0 0Color 20 0 0

On the following images, the same ellipse with same colors has been applied on the cloud layer. The only change is the Alpha Diff mode.

Color 255 0 0Color 128 0 0Color 20 0 0