Basically, copyediting takes place earlier in the publishing process than does proofreading. In fact, the copyeditor creates a style sheet listing the references relied upon (usually a specific dictionary and style manual) and the decisions made on spelling, punctuation, capitalization, hyphenation, presentation of dates and numbers, etc. This style sheet is specific to your manuscript and can then be referred to by the proofreader when checking the project before it goes to print. For example, if you prefer a less common spelling of a word and have used that spelling consistently throughout your manuscript, the copyeditor will note this in the style sheet and the proofreader will see that. Otherwise, the proofreader might be tempted to change the spelling, which might introduce inconsistency and even increase costs since changes at the proofreading stage are often more expensive.