According to today's headlines, the outline level is usually used to identify the hierarchical structure between chapters and paragraphs in a document. In formal documents, the outline level usually appears in the table of contents. For example, a paper with multiple chapters, each chapter will be assigned a specific outline level, such as "Chapter 1", "1. 1 background introduction" and so on.

By setting the outline level, readers can clearly understand the logical relationship between chapters or sections in the document, so as to better understand the main idea and argument of the article. At the same time, the outline level can also help readers to quickly locate and review the key information in the document and increase the reading effect.