Typically words. In all their forms and formats. Language, protocol, and communication too.
Proofreading is technically the final polish of a project, when I zero in on grammatical, spelling, and other mechanical errors. I always make sure that the entire piece makes sense, reads well, and is consistent in tone and style.
Conversational and writing skills boosts. For those who want to expand their facility with written and spoken English, whether English is a new language or an old friend.
Copy editing is called for when a project needs a bit more work. Text may be changed to improve the flow and overall quality. Errors are corrected, and I use my expertise to ensure that everything makes sense, reads well, and is consistent in tone and style.