What goes on it, and why the manual cannot cover it
A style manual settles the general questions. It cannot settle the ones that belong to this document alone.
Names are the largest category. Every person, place, company, product, and invented term goes on the sheet, spelled and capitalized the way this manuscript spells and capitalizes it. A manual has nothing to say about whether a character's nickname takes an apostrophe, and no manual ever will.
Numbers come next, because the manual usually offers a general rule and an alternative, and a document has to pick one. So do dates, times, measurements, and currency.
Then the local decisions: which terms are capitalized, which are italicized, which abbreviations are used and after how many appearances, how headings are cased, and how a recurring phrase is punctuated. None of that is in dispute anywhere. It simply has to be decided once and then held.
The last section is the queries and their answers. A question settled on page 40 should not be asked again on page 300, and the only thing preventing that is a record.
The four things it does after the edit
The first is protecting the author from a second bill. A proofreader working without the style sheet has to rediscover every decision, and will query things that were already settled.
The second is making a series possible. A book edited with a style sheet has, without anyone intending it, produced the document a second book in the same world needs on day one.
The third is surviving a change of editor. Editors Canada's fundamentals ask that queries and author letters be clear, tactful, succinct, and actionable, and that changes be made while maintaining a consistent voice, tone, and register.3 A new editor inheriting the file inherits none of that unless it is written down.
The fourth is settling arguments. An author who disagrees with a decision can see when it was made and what it applies to, which is a shorter conversation than one about whether it was made at all.
Why it outlives the software
A word processor's grammar and style settings are not a style sheet, and Microsoft's own documentation explains why. The rules can be selected or cleared, and the settings apply to all the documents an author edits rather than to the current one.4 A record attached to the software is a record attached to every document at once, which is the opposite of what a manuscript needs.
A style sheet is a document. It travels with the manuscript, it can be read by a person, and it does not change when someone updates an application.
What to ask for, and when
Ask at the point of quotation rather than at delivery, because an editor who is building one from the start builds a better one than an editor assembling it afterward from memory.
Two questions settle whether a quotation includes it. Does the price cover a style sheet returned with the manuscript? And does it list decisions as well as spellings? A list of preferred spellings is useful. A list that also records why a decision was taken, and what it does not cover, is the document worth having.
EditFast returns the style sheet with every copyedit and proofread, records the queries raised alongside it, and works to whichever style authority a client has chosen rather than imposing one. Everything a client sends is treated in confidence, including manuscripts, style sheets from previous editors, and correspondence.
An author with a style sheet from a first book has already paid for most of the consistency work on a second. An author without one is paying for it twice, and usually does not know that is what the second invoice is for.