There are steps in the correction process in which it is impossible to differentiate between the different levels: there is so much pending that needs to be checked and considered. At most, we assume priorities and choose the first battles. The syntax has priority over the orthography; the communicative intention over the syntax. As we move forward, the first drafts, by dint of hammer blows and woodwork (as Gabriel García Marquez identified this process), becomes something legible, and who knows?, even enjoyable. This way, the correction starts focusing more on the details and less on the structure.
Every editor specializes in some level. There are some that love the initial work, with structure and strategy challenges; other specialize their ability to detect the smallest details in the writing: they detect the misprint, the lack of graphic accent, the badly placed period, the missing line and the excessive dashes in the almost final versions…
Finally, when the text has gone through many correcting hands (from the author to the proof-reader, from the proof-reader to the style editor, from the style editor to the fact-checker), is time to make the decision of ending the correction process. Even if we know there can always be malicious pixies camouflaged between the paragraphs willing to leave an error in the first page we will open when we take the first copy from the printer.
Being aware of this, we as editors still keep doing our job, because every error found means one less, and it also means one segment more of the straight line we need to set our product closer to the ideal we compare it with.