Our most important documents should go through multiple revision stages.
In the drafting stage, you need developmental edits from subject matter experts or editors specializing in manuscript evaluation and developmental editing.
If you get feedback that your writing needs to be improved, you’ll want to look for heavy copyediting. I like doing this kind of editing! It’s focused on idiomatic and grammatical American English.
When you’ve integrated the feedback from all of your mentors and readers, you’ve done the best you can on the writing, and honestly you’re a little tired of looking at the thing, that’s a great point to bring in a copy editor.
When you think it’s really done and ready to submit? That’s when you want a proofreader to check it one last time.