Copy editing is in some ways the big brother of proofreading, and the little brother of line editing. It is concerned with the style and clarity of the prose used, and not just spelling and grammar, but takes a much narrow view than line editing, and does not include large rewrites to improve pacing.

Note: There is no consistent terminology in the industry. Many places use the terms "copy editing" and "proofreading" interchangeably. Other places don't differentiate between "line edit" and "copy edit". I offer all three as different services.

Do

• Make sure the draft is ready for a copy edit. It should be completely sound in terms of structure and pacing, and not have major stylistic problems.
• Use a copy edit as a polishing pass after the text has undergone revisions, such as after incorporating feedback received for the first draft.
• Use a copy edit when a previously-edited text had to be reworked, such as when a copy had to be redone to fit smaller available space.
• Use a copy edit if you're confident the text is in a good shape and will not need major stylistic improvements.

Don't

• Don't expect a copy edit to be able to fix a text with structural issues. No amount of stylistic fixes can help a book with an unclear plot and underdeveloped characters. Consider a developmental edit or manuscript evaluation instead.
• Don't use a copy edit in place of a line edit. If you have to choose between the two, choosing the line edit is almost always the right call.
• Don't skip proofreading after a copy edit. Although the goal is always to produce an edited copy that's completely polished, it's very hard to ensure no typos managed to sneak in without a dedicated pass which only looks for mechanical errors.