What do you mean the TOC in the metadata.opf file?
The OPF has several things, things, none of them is a proper TOC:
- There's the spine, where all the elements are listed in reading order. This is not a TOC, it's an ordered list of files, no titles or links are implied here.
- There's the guide, where some links can be given to parts of the book (preface, cover, copyright...). This is not a real TOC either, just a collection of pre-defined bookmarks, which may be used or not by the reading system.
The real TOC is the NCX file. This file has all the links and titles you'd expect to find in a TOC, and most reading systems have a way to show these and select them. A reference to this NCX file must be included in the OPF manifest, and in the "toc" attribute of the spine.
[EDIT: DMSmillie explained it more thoroughly, that's what I get when I make a pause in the middle of writing a post
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