Created attachment 126536 [details] Mock up for 'must not have this tag' checkbox. SUMMARY Currently when you want to exclude certain tags you can use 'must not have this tag' function to do so. While this is very useful, the checkbox looks the same when tags are selected normally, so you can't differentiate them. I would suggest something like a minus inside the checkbox. I've made a quick mockup of how it would look like and attached it.
I like the minus sign for "Must not have". Additionally, the checkmark could be replaced by a plus sign for "Must have". Perhaps color (green/red) could also be used to indicate the status of each tag.
Created attachment 133733 [details] Example of minus sign within checkbox representing mixed state
Alternatively, a checkmark "✓" and "X" mark could represent the two states, rather than plus and minus. Minus and/or plus signs within the boxes have the potential to look like expandable hierarchy disclosure widgets, which often also look like + and - within squares: https://docs.microsoft.com/en-us/windows/win32/uxguide/ctrl-progressive-disclosure-controls#plus-and-minus-controls A minus sign within a checkbox is also sometimes used to represent an indeterminate or mixed state, as when some but not all of an object's child objects have an option set. Example of this use from Gmail (when managing labels on multiple selected messages) is attached. Here is an example of a three-state checkbox using both color and ✓/X marks to indicate filter inclusion and exclusion, very much like the use case for tag filters in digiKam. A runnable CodePen example is included: https://medium.com/@carsonf92/introducing-the-three-state-checkbox-1b6f00b6ec89