A final polish. Or, a detective's dream?
Let me just pull out my Sherlock hat and magnifying glass. I will make sure that no errors have been missed and that none have been introduced during layout. Nothing shall get past me!
Whether in isolation or against a previous format, we are taking a final format and checking it over. I look for minor mechanical errors and consistency issues (I'll just be over here, cross-referencing facts and information 'til my heart's content). I ensure that links work and I check page numbers. It's the tiny details that can make or break the final impression. Proofreading isn't an alternative to editing. Instead, it comes after, to make sure everything is just right.
A lot of the time, I am using that magnifying glass to find the errant comma and sneaky typo. Aha, Sir! I've got you! Deleted, you shall be. Wait—that's not Sherlock, its Yoda.
PS. Did I just use a fancy em dash? Why yes, yes I did.
*Strikes cool pose