Nadine gives us a demonstration as to how to reach the darkest value for graphite ... what we call a value 9. Our value scale runs from 1 to 9.
1 is our lightest light for the capability of our medium
9 is the darkest dark that we are capable in out medium
These numbers are not absolute measurements (like they are in the Munsell scale). These numbers speak to the capability of the medium we are working in and simply serve as approximations for discussing relationships.
This value scale looks different according to the medium you are using (paint's version of "9" is much darker than graphites version of "9"). Value is all about relationships.