If you go to infection/skin/cellulitis, it directs you to see cellulitis, by site
You can find further informat regarding the term "use additional code" in section I, conventions, general coding guidelines and chapter specific guidelines in your ICD 9 book. It will then direct you to "multiple coding for a single condition". Under which it states that these notes are found in the tabular at codes that are not part of an etilogy/mainfestation pair where a secondary code is useful to fully describe a condtion.
It does not state it is required to report the primary code.
Hope that helps, happy coding!