When using drawn masks, there is a “toggle polarity of drawn mask” button that shows either a minus or a plus sign and also a dropdown menu called “invert mask” that can be set to “on” or “off”. They are marked with red in the screenshot below.
As far as I can figure out, these two do exactly the same. I have tried every combination of masks and these settings I can think of without being able to make them behave differently. Therefore I wonder if there is something I have missed. So is there a difference? and if so, what is the difference? I couldn’t find anything about it in the manual, although the topic of masking is very well covered.