Line editing is a scene- and paragraph-level edit that helps with voice and style.
e.g. Identifying sentences that are too wordy or paragraphs that are too confusing.
You may notice that line editing overlaps some with the developmental edit and some with the copy edit.
Copy editing is the process of going through a novel and correcting any grammatical mistakes, factual errors, and story inconsistencies or continuity errors. This is a deeper, more thorough, and less constrained edit than proofreading.
e.g. Rooting out dangling modifiers, finding style issues per Chicago Manual of Style, formatting dialogue properly, and nixing other grammatical errors.
The middle stage is less about ensuring your story is viable and more about how you convey that story to readers. The biggest benefit is in improving your readers’ experience with actionable advice from an industry professional.