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Developmental Editing, sometimes called Substantive Editing, is more in-depth and focuses more on the actual content, in addition to the general proofreading. So, they are looking for plot or information inconsistencies, asking questions to clarify specific concepts and definitions, giving suggestions for areas where a different or additional example would be useful or where less detail would be better, looking at the overall structure and order of the book, restructuring sections, paragraphs, or sentences to be more clear, giving suggestions for wording changes, and can even include helping you come up with a title, cover design, and formatting suggestions. These editors want to keep your original messaging and tone but help clarify the content.
Copyediting is looking at overall clarity and consistency of tenses, perspective, formatting, and style, as well as general proofreading for grammar, punctuation, spelling, syntax, and diction, while maintaining your original content, meaning, and tone.