Heavy copyediting is often appropriate for early drafts and manuscripts that have received minimal review or revision. These typically contain more errors and inconsistencies, thus requiring more intensive editing. Heavy copyediting is slower and demands more organization and experience on the part of the copyeditor, hence its higher cost. Manuscripts that need heavy copyediting frequently require fact checking as well.
For longer manuscripts, different chapters or sections often receive different degrees of copyediting. For example, academic articles and dissertations that need only basic copyediting of the body text frequently require heavy copyediting of their citations and bibliographies. In such cases, the appropriate rate is charged for each section.
Joshua Boydstun 