Help+Manual's conditional output features can create a wide range of variants of your project for different purposes. However, it is difficult to create versions with TOCs that have very different structures using only conditional output. The multiple TOC feature solves this problem. It allows you to create completely different TOCs with a different structure and different entries for different purposes.
The entries in your additional TOCs can point to the same topic files as in your main TOC. Or they can have their own topic files that only exist in that TOC. When you publish you choose which TOC you want to use, and only the topics referenced by that TOC are exported – in some cases along with topic files linked to in those topics, to prevent dead links.
See Publishing projects with multiple TOCs for details on selecting specific TOCs when you are publishing your projects. Also see the Multiple TOCs tutorial project that you can access directly in the Welcome page at the top of the Project Explorer on the left.
Preventing dead links (Conditional Output)
Publishing projects with multiple TOCs (Publishing)