Field called AND/OR (c)

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I am working on some Report Improvements (i.e. fixing reports that don't consistently run) and working on one that searches thread key words. I stumbled upon the field search_and_or as a search filter, and I know I read a KB article/help page/forum post about what it did, but for the life of of me, I can't recall. Is anyone familar with what it does and how it helps in report. 

I see its a menu item, but would like to understand the impact of the choices.

Thanks!