What exactly is this No? Is there any other use of it other than graphs? Thank you so much. I am not trying to cram or anything it's just that I took a course online and a lot of the time it focused on concepts rather than notation. Sometimes when notation comes up I don't understand it.(seen in precal textbook)
It's not your fault, this is not a notation and it's not standard at all.
I think the book is trying to tell you that $g$ is not defined at 8, by telling you that there's no value $g(8)$, showing with that vertical arrow where it would have been if it was defined.