Copy-editing usually demands greater intervention in the text to ensure coherence and clarity.
In addition to a thorough proofread, some of the below may be necessary:
Flagging up repeated words or tautology
Checking idiom (accuracy and whether correct in context)
Suggestions for rephrasing if something seems ambiguous
Querying possible missing content
Depending on the publication, suggestions for restructuring lengthy paragraphs to make information clearer or to improve flow.