on Writing Forward:
We usually think of editing as a process that cleans up our grammar, spelling, punctuation, and sentence structure. But developmental editing is a little different. A developmental editor looks at the content and structure of a writing project, paying little (or no) heed to the mechanics of the language.
Developmental edits are for projects that are destined for submission or publication, usually a long-form project, such as a novel. As the term implies, this edit is done while the project is still in development; in other words, it’s not part of the polishing process that is usually associated with editing.