When it says "an A plot or a plot A" it is introducing the concept. Later, it uses "plot A" as a name, assuming you are comfortable with the concept already. But they both refer to the same thing.

The difference lies in specificity and how the terms are used: 1. "a plot A" * Refers to a generic example or concept of a "main plot." * It's less specific, implying that it's one of many possible plot structures. * Example: "A plot A usually focuses on the protagonist's journey." 2. "plot A" * Refers to a specific plot in the story being discussed. * It's more direct and assumes the reader knows which plot you're talking about. * Example: "Plot A in this episode focuses on Aang's training." In your sentence, "plot A" works better because you're talking about a specific structure in Avatar.