A sub-editor is NOT the editor’s assistant, or SUBstitute editor.
A sub-editor is a journalist who edits (corrects, changes and improves) articles. He probably never writes original articles for his own publication. But he is an expert on grammar, spelling and good writing.
He is also familiar with his publication’s policies and makes sure nothing creeps in that the paper wants out (such as four-letter words spelt out in full).
While not a lawyer, he can spot dangerous material that needs the opinion of the top person (the editor or publisher) or a lawyer.
Sub-editors are also responsible for layout, that is, where pictures, captions, headings and the story itself FITS IN on a page or pages (what the pages look like).
On small publications, often the editor does all the sub-editing or “subbing” as it’s known in journalism. Sub-editing is a vital function in a professional publication.