It's determined based on which of the categories/terms are excluded from the search. If a term is excluded, it will appear as unchecked. You can exclude them on the Advanced Options panel: https://i.imgur.com/EOnJPq2.png
There are similar exclusion boxes on the Frontend search settings panel, but those only determine which of them should be visible at all.
Best, Ernest Marcinko If you like my products, don't forget to rate them on codecanyon :)