Article on executive privilege
A short piece in the Washington TImes. I handled FOIA requests when I was with the government. At least at my level (career staffers rather than the POTUS) the rules were essentially that you could withhold deliberative materials, where staffers advised on or debated what to do, thus ensuring they could evaluate things without being chilled (no agency will, of course, agree that that applies to private citizens). But it could not withhold factual portions, parts that described happenings.