What is the “with_front” rewrite key?

I have the following line in a register_post_type function:

'rewrite' => array('slug' => 'Newsletters', 'with_front' => false),

I understand what the line does in general (adds “newsletters” to the slug) but I don’t specifically understand what the ‘with_front’ part does. I have to admit, I don’t really have a problem that needs solving, but I’m suddenly curious….

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