Zend certified PHP/Magento developer

What does Category’s attribute is_anchor do?

I’m doing some sample tests from Magento 2 certification. There’s this question (screenshot) I don’t understand.

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The correct answer for this question is A “This is the default category for a website”, but according to my knowledge (I have been working with Magento for many years), if we turn Anchor on for a category, it’s just enable the layered navigation for this category.

I wonder why the correct answer is”This is the default category for a website” when is_anchor is set to 1? can someone please explain?