Animation of how a custom style is added to a TOC.
After identifying the styles used in a document, you set up the TOC to collect those custom headings along with Heading 1, Heading 2, and Heading 3.
On the References tab, click Table of Contents, click Insert Table of Contents, and then click Options. Under Available styles, find the name of the style that was used for the chapter titles or headings. Next to each style, type a number from 1 to 9 to indicate the level that you want the custom style to appear as.
Note Because your document also uses Heading 1, Heading 2, and Heading 3, you'll also see those styles in the list of available styles marked with a TOC level.
Click Play to see how a custom style is added to a TOC.