Structural editing is the practice of shaping how a draft holds together before anyone starts arguing about wording. It covers the order of the sections, the promise each heading quietly makes, and the two or three places where a reader is asked to take you at your word. Working outlines like Cadenza, Rivet, and Longform keep that skeleton visible while the sentences are still moving.
Line editing works much closer to the surface, and is responsible for rhythm, verb choice, and the small hinges that stop a sentence from folding in on itself. It leans on a handful of unglamorous habits: reading the paragraph aloud, deleting the first clause, and letting the whole thing sit overnight before it ships.