Custom Styles

Each table of contents entry has a style (TOC 1 to TOC 9) that determines the formatting of that entry.
Each TOC entry is also assigned a TOC level (1 to 9) that indicates the TOC style used for that entry.
You can create your own styles or use one of the built-in formats of TOC entry styles in word.

SS examples

A custom heading style is simply one that you have created yourself.
If you use your own custom heading styles to format the headings you want in your TOC you can choose the TOC level for these styles in the Options dialog box (link)