Outside the book publishing industry, the term proofreading can have many different meanings. When it comes to books, though, it is a very specific stage. Proofreading is the last thorough, page-by-page look at a book, usually done simultaneously by the author and a freelancer, before it is sent to the printer for folding and gathering.
Proofreading is not the time for the proofreader (or the author) to change voice or style or decide that figures should be moved. Every change after first pages are typeset will cost the publisher money, whether the designer is in-house or freelance. The book’s project manager will go over the changes suggested by the proofreader, and anything that is not absolutely necessary will be crossed off in blue pencil with a big STET (let stand).
But some changes are unavoidable. If the running heads in one chapter are wrong, they must be changed. If a name is misspelled, it should be corrected. Widows and orphans should be fixed, and every caption should be scrutinized to make sure it fits its figure. There are also some changes that are expected, such as page numbers added to the table of contents and list of figures. Proofreaders will also add page numbers to the running heads in the notes section.