In line-editing, we look at the style of writing, page by page, paragraph by paragraph. We make sure the right style, words, and tone are being used.
Copyediting is more granular: it looks at the way words are spelt, and whether phrases are grammatically correct and sentences properly punctuated.
The rule I adopt in copyediting a client’s work is: don’t mess with it. If the language is correct and clear, it doesn’t need improvement.
As a writer, you don’t want someone changing your words because they think it “sounds better”.
You want the things that are actually wrong or confusing to be put right.
You want the grammar, spelling, and punctuation to be correct.
You want accuracy and consistency.
No mistakes to distract the reader.
That’s the way to make sure they will enjoy what you’ve written.