A family of Microsoft word processing software products for creating web, email, and print documents.
The headings that are displayed in a table of contents are controlled by the items in the field code of the TOC field. To see the code, press the keyboard shortcut Alt+F9 (or Alt+Fn+F9 on some laptops). That's a toggle; if you press the same keys again the code will be replaced by the full display. The code should look something like this:
where the part following the \t switch tells Word to display any heading formatted with "Special heading 3" style in the TOC at level 3 (using TOC 3 style). [This and other parts of the TOC field syntax are explained at https://support.microsoft.com/en-us/word/field-codes-toc-table-of-contents-field.]
To get this kind of code in a custom TOC, click References > Table of Contents > Custom Table of Contents. In the dialog, click the Options button. In the next dialog, scroll down the list of available styles to find the style you're using for your special headings. In the "TOC level" box to the right of that style, enter the TOC level where you want these headings to appear [this doesn't have to be the same as the style's Outline Level in the Modify Style dialog].
When you click OK in the Table of Contents dialog, if there's already a TOC in the document you'll be asked whether to replace the existing TOC -- generally you'll say yes.
The result of the field code shown above is this: