A document can contain multiple Tables of Contents. Each Tables of Contents can refer to a bookmarked range and/or a specified set of Styles. The default Tables of Contents that Word creates comprises entries from the Styles Heading 1 - Heading 3, for the whole document.

So, for your situation, you might have a 'main' Table of Contents that picks up just the Chapter headings (presumably in Heading 1) for the whole document.

In turn, each Chapter would have a bookmark applied to its entire range. Then, immediately below each Chapter heading in the body of the document, you could have another Table of Contents that refers to, say, the Heading 2 & Heading 3 Styles for just the bookmarked range. Within the chapter, the Heading 2 & Heading 3 Styles would be used for the main and sub headings, respectively.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]