The so-called main line refers to the main content of a novel. For example, in some novels, the protagonist has a blood feud at first, then the protagonist does a lot, and finally the protagonist succeeds in revenge. Then revenge is the main line, the clue is the foreshadowing of a plot, and it is a writing technique. Want to be a detective Conan, the author wrote Conan to solve the case, but at the beginning, he didn't write where the deceased was, but where Conan and others went to play, which was also a clue. I hope it helps you.