Digital Light & Color

Clicking Add or Subtract buttons seems to do the same job as a mouse click, so it is not possible to switch, for example, from Add to Subtract and vice versa without at the same time adding or subtracting from where the point momentarily is.

Example:
I used Flood Fill to mask a region in the image. Then I wanted to subtract part of the masked region, so I switched to Subtract but this automatically subtracted part of the masked region without me even moving the mouse where I wanted to subtract. Switching back to Add reselected the subtracted part, but there I got stuck without being able to continue using Flod Fill for subtracting unwanted part of the masked region. I used Paint to subtract instead. There is no problem switching between Add and Subtract with Paint.

The pdf file says:
.....If you make a mistake and accidentally mask part of the image you didn't want to include,
you can switch to Subtract mode and each new region you mask will be removed from the
current mask, so you can just outline the part you want to remove, and it will be deselected.....

Each new region.... but it evidently first removes the last region masked.
Did I get it wrong? Or is it necessary between switching to always perform pending operation first?

Until you finalize the mask by clicking the red running man button, changing the mode applies to the current mask operation. This lets you see the effect of the different modes without having to undo and re-apply the operation. Once you finalize, the mode button apply to the next operation.

Mask operations such as the freehand lasso auto-finalize when you release the mouse button after dragging, so the next mode change applies to the following operation. This is the situation described in the help file.

I have updated the documentation for the next release for be clearer on this point.

OK. Thanks for the clarification. I was not sure whether this difference from other masking tools, such as paint, was intended.

It is intentional -- the freehand outline and paint tools auto-finalize whereas all the other require finalization.