The first-level title is also called the title, which generally refers to the title of the paper.

The second-level title, also called chapter title, is the biggest title in the paper, such as the first chapter. .............

Three-level titles, also known as chapter titles, are the titles subdivided in a chapter of the paper, such as the first section. .....................

Example:

Topic of Linguistic Thesis: On the Evolution of Chinese (the first-class topic No.2 is bold and centered)

First, the evolution of Chinese word meaning (the middle of the second-level question Xiao No.4 is bold)

(a) Transfer of meaning (third-level title, No.4, in bold)