A developmental editor is an editor who works with authors on the overall structure and development of their book. This can include helping to shape the book's concept, providing feedback on the book's content, and working with the author on refining the book's structure.
Developmental editors often work with authors from the early stages of writing a book, and can be involved in the book throughout its development process. This can include working with an author to help them develop a proposal for a book, providing feedback on drafts of the book, and working with the author on revisions.
Developmental editors can also provide guidance on the publication process, and can help an author to find a publisher for their book.