The secondary title is the subtitle of the paper. The title is the title of the paper; The first-level title is the first point of discussion, such as preface. The word is usually one, or 1,; A secondary heading is a sub-topic under a primary heading, such as (1) (2) ... or 1. 1, 1.2 and so on. The main purpose of setting subheadings for expanded materials is to clearly show the level of the article. Some use words, which generally express the central content of this level; Some also use numbers, which only indicate the order of "one, two, three", and play the role of connecting the preceding with the following. It should be noted that no matter which form is adopted, it is closely related to the content of the hierarchy and the close relationship between the upper and lower levels.