MacGuffin
An example of a detail that is not a MacGuffin is the Ark of the Covenant in the movie Raiders of the Lost Ark. Even though the Ark is important to the motivation of the movie's hero, Indiana Jones, and the villains, the Nazis, the Ark's importance becomes more clear in the later half of the movie, and it cannot be replaced by anything else because the Nazis' misuse of the Ark was the direct cause of their death at the climax of the movie.