The figure calls it an imaginary line, but it isn't imaginary. It really is only "imaginary" with respect to getting a lot line, because it's not a lot line. But the space that establishes exterior wall fire-resistance ratings and allowable opening percentages relies on the distance to this line, as https://ibc-container-zubehor64207.get-blogging.com/36836476/the-smart-trick-of-seecontainer-kaufen-that-no-one-is-discussing