I or MCT (epitomized by the POV shot in a film) - Places the audience in the story. A special relationship exists between it and Swain's Reaction Unit. This Throughline is the most likely surrogate for the audience because it is the most rounded depiction of any character in the entire story. As someone said earlier, it puts you in the story.
You or ICT (epitomized by the Reaction Shot in a film) - Acts as an alternative perspective. The only difference is that the audience cannot be inside "you's" head. The audience may form a connection, but it is still limited. In real life, this is like a best friend, mentor, parent. No matter how close, involved, or influential this person is in your life -- he's still not you. Will be in Swain's Motivational Unit.
We or RST (epitomized by the Two Shot in a film) - Is different in scope because of the commentary that the shot/signpost makes is about the relationship and how it changes.
They or OST (epitomized by the Long Shot in a film) - The big picture. The reason that everyone is involved.
I think storytelling uses reflection, similies, metaphors, comparisons, contrasts, parallels, etc. to get the most out of each scope of shots. For example, an MC's state of being or state of mind can be shown in the environment (Long Shot), the reaction of a friend (Reaction Shot), and the different relationships (Two Shot). But, it is just a reflection.
For example, check out Jim Hull's article:
The bolding is my own emphasis, but it serves to show how a Throughline can bleed over and make commentary on another scope/POV of the story:
The other side argues that the Empire’s illegal boarding of a diplomatic ship begins the story’s investigation into a process known as problem-solving. They see that opening event as an essential component to a holistic understanding of what Star Wars is really all about. Remove it, and the rest of the film simply becomes sci-fi eye-candy. Why?
The Empire’s aggression in those opening scenes has a meaningful connection to what is going on inside of Luke personally.
I think that Scope and POV are thought to be interchangeable and this causes some problems. Each of the Throughlines involved scopes. But only one of them involves a complete POV: the MC.
I think what else can be confusing is the fact that a story is never just these Throughlines. Maybe the Throughlines represent the bones, mind, heart, conscious of a story, but there's so much more.
For example, you have the RST in a story. But in most stories, there are bit players (characters that don't represent a Throughline specifically) that create a number of different relationships that can illustrate specific signposts that you want to explore in the OST, MCT, and ICT. Even the RST can be illustrated by the inclusion of bit players that act as a reflection, comparison, contrast, or commentary of the RST. I think of these as the connecting tissue or redundant systems in a body.
The environment, surrounding bit players, reactions, commentary, etc. can all be used to point out and encourage audience understanding of a relationship, scope, or POV.