I've been reading Elizabeth Lyon again, and I'm trying to gain some insight into what she means by these elements of storytelling. I write using all of these elements, but perhaps a closer inspection will improve my understanding of just what it is I'm doing. Maybe you want to ponder them, too.
I tend to think of stories and writing in terms of scenes. Long or short, busy or quiet, conversational, pensive, descriptive. Everything is a scene, regardless of content or what it is intended to show. So these terms really just refer to different kinds of "scenes".
By Ms. Lyon's definition, "scene" refers to action. To her, a novel or story is a patchwork of Scenes (action) strung together with other segments, ie Set Pieces, Sequels, and Segues. (Also Subtext and Shortcuts, which I'm leaving out.) The idea is that you use each element as needed, of course.
I'm not quite at my Eureka moment with this, as you can see. But here's what we know:
"Scene" refers to an action sequence.
"Sequel" is a section that shows a character's reaction to a scene, in terms of both emotion and thinking, and winds up with another action based on whatever decision is reached.
"Segue" is a transition between two time frames or two types of action.
"Set Piece" is a moment of high tension, which seems to occur at a turning point in the story.