FifthView
Vala
We'd had a discussion about throughlines not too far back, I think maybe before you returned to MS: Throughlines.
It's what the story is "About," capital "A." Everything else introduced in the story will be attached to that and circle that, to serve the throughline, although some things more obviously so and others maybe less obviously (for the reader) until later.
You can also think about the promise you make to the reader early in the story, a promise that you fulfill by the end of the story. So it's also related to the principle conflict and the primary hook.
It's what the story is "About," capital "A." Everything else introduced in the story will be attached to that and circle that, to serve the throughline, although some things more obviously so and others maybe less obviously (for the reader) until later.
You can also think about the promise you make to the reader early in the story, a promise that you fulfill by the end of the story. So it's also related to the principle conflict and the primary hook.