In the table of contents above, each chapter uses a heading style, so there are four sections. When you insert the table of contents, it will create a section for each heading. If you apply a heading style, you're telling Word that you've started a new part of your document.
Styles also serve another important purpose: adding a hidden layer of organization and structure to your document. You can use Word's built-in Style Sets or create your own. Style Sets are affected by the overall theme, font theme and color theme used in your document. While you can create your own styles and use them as headings, or adjust the formatting of headings on the fly, you can also change the default styles if you like. Styles define the font family, size, color, and more. If you've already read our Applying and Modifying Styles lesson, you know they're an easy way to add professional text formatting to different parts of your document. Style Sets are designed to include heading styles (such as Heading 1, Heading 2 and Heading 3), the Normal style, and a few other built-in styles (but not all styles). Word provides built-in styles for several different levels of headings and subheadings: Heading 1, Heading 2, and so on.
However, with the right formatting, Word can create and update a table of contents automatically. And if you ever decide to rearrange your sections or add more information, you'll have to update everything all over again. You can also create a custom style and assign it an outline level so the style will be treated as a heading. You could create a table of contents manually-typing the section names and page numbers-but it would take a lot of work.