“Tone” is a word that seems to come up every time you write anything in school, university, absolutely anywhere. You are told to write creatively, write formally, write as if you’re talking to someone who knows nothing at all. And yet, they don’t really tell you what it means to have that tone. You’re the one who has to figure that out.
You want to write with a tone, proofread with a tone, and copy-edit without breaking that tone. You have one hint - what kind of document you’ve been set to write or proofread, or copy-edit. After that, you have to figure it out. Do you use contractions? Does the writer know what you’re talking about? Are you expected to explain every little thing, or leave certain obvious facts out?
If it’s an essay, or a thesis, or something equally factual and analytical, it usually means to spare most contractions (some are more acceptable than others) and to use a formal tone. You know what the audience wants. It wants you to inform them, to tell them that this is what you were looking at, and this is what you found out.
A work of fiction is much harder, because the tone can be very different. Is it descriptive? Is it colloquial? Is it past or present tense? Is it first, second or third person? If it’s the latter, what kind of third person is it - limited, or omniscient? There is really no way to write fiction with the wrong tone, unless you use it to inform. The audience almost always reads fiction to be entertained. And because no one can entertain everyone in the world who reads their book, they have to pick and choose their tone by targeting the audience they want.
As a proofreader or copy-editor, your job (and my job) is to identify the tone before you start editing. And to make sure that not only are they keeping the tone consistent throughout the whole thing, but that your edits (especially as a copy-editor, where you’re playing with words more) never break the tone.