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DOCX files from Microsoft Word 2007 and newer are supported.
For example, in the calibre viewer, you use the Show Table of Contents button to see this To C. This is because the MOBI format, alone amongst mainstream e-book formats, content To C at the end of the book.
So it is recommended that you leave the generated To C alone.
DJVU support is only for converting DJVU files that contain embedded text. This Table of Contents is part of the main document flow and can be styled however you like. A metadata To C is a To C that is not part of the book text and is typically accessed by some special button on a reader. In the MOBI format, the situation is a little confused.
The first thing to realize is that most e-books have two tables of contents.
When converting to MOBI, calibre detects the in the input document and generates an end-of-file To C in the output MOBI file.
You can turn this off by an option in the MOBI Output settings.