Set WWR by Facade - does the "don't alter spaces not included in building floor area" option work?

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My hope using this was that if, on the spaces tab, I unchecked "Part of Total Floor Area" I could exclude certain spaces from the overall change in selected facade WWR, but this doesn't seem to work. Am I misunderstanding the intent behind what this check option is supposed to offer? This would be a great feature as the WWR measure is somewhat unwieldly otherwise.